How To Be More Friendly And Social
Here are some general pointers on how to be a more friendly, social person. A quickie definition of 'friendly' would be being nice to, and interested in, other people. I'd define 'social' as spending a fair amount of time with other people and enjoying it.
The advice below talks about specific actions and overall dispositions (e.g., be positive about people, be interested in others) at the same time. The two are connected, but you don't have a ton of control over your disposition towards other people. If you're not in an outgoing, chatty mood at the moment, or if you're more reserved in general, that's just the way you are. You can't flip a switch and feel warm and loving towards everyone instantly. However, you can still keep some of the specific actions in mind, and they may be all you actually need. For example, if you're at work and you find you're keeping to yourself that day, you can remind yourself that you should go join your coworkers and see what they're up to.
Also, the ideas here don't suggest that you need to turn into a phony, or a needy suck up, or an over-the-top caricature of a 'friendly' person. They should be thought of more as background attitudes that subtlety influence how you approach interactions with other people. A serious macho guy and a more affable, breezy type could use the same basic concepts and still maintain their own personality styles. Be fairly low key about implementing the points below.
Start conversations with new people
If you've recently been introduced to someone, or you see some new people around, go up to them and start a conversation. Even saying hi, asking for their name, and going, "Cool, nice meeting you. I'll see you around later hopefully" can be good.
Chat back to people who try to talk to you
Have you ever tried making pleasant conversation with someone you've run into, and they blew you off by giving one word responses and obviously looking like they don't want to be spoken too? You probably walked away thinking they were pretty unfriendly, even if you intellectually knew they may have had a reason for being brusque. If someone is trying to chat with you, make an effort to give them something back in return.
Take time to talk to people you already know
If you see someone you know, then go over and see what's going on with them. No real reason, just because. Catch up with what they've been up to lately, or just talk about whatever. Keep in touch with your friends. Stop and chat to your coworkers when they're not too busy. Maintain your relationships and show you're interested in the other people. If you see someone you know, don't avoid them because you don't feel like talking, or pretend not to notice them because you're worried the conversation will be stilted. Go up to them and shoot the breeze for a few minutes.
Invite people to do things with you/the group
Be fairly loose and generous with your invitations to people. Be the one to invite people out rather than waiting for them to come to you first. Don't feel you have to know someone for a long time either. If you seem to get along with them then why not ask them to do something? If you like your new coworker or classmate, ask them if they want to grab a drink later, or come by your place to chill. If you run into a friend downtown, and neither of you is doing anything, ask if they want to grab a bite to eat, or if one of you is busy, suggest you get together later some time. Ask the new guy in your apartment if he wants to play pool down the street in an hour or so. Don't feel you have to know someone for a pre-designated amount of time before you can hang out with them.
If everyone at work is going out on Friday evening then ask anyone who may not know if they want to come along as well. If you're meeting some friends later that night, ask your new acquaintance if he wants to join you. If you run into a buddy on the street for five seconds, tell him that you're going to be a Dan's place later if he wants to drop by. Of course, when you throw invitations out like this, they won't always be accepted, but that's cool.
Make an effort to bring new people into the fold and make them feel included
If you're out with your longtime friends and there's a new person there, take the time to talk to them a bit, rather than being more aloof and expecting them to make the effort of getting to know you. At the end of the night mention that everyone is seeing a certain concert in the next two weeks if they want to come. If there's a new person at work, fill them in on the general goings on, and let them know everyone in your department usually grabs lunch together at 12:30. Mention that you and three other people usually play football on Thursday evenings if they want to join in.
Go to where the people are
If you're at work and everyone is going out for lunch then go as well. If they all eat lunch at a certain time and place, then eat lunch then too. If you're at a party and everyone is talking on the front porch, go join them. If you're at a bar and everyone is hanging around on the couches downstairs, then you may as well be there too. Show you want to spend time with the people you came with. And once you're there, join in whatever they're doing. Don't hang back and get lost in your head.
Make nice little gestures towards other people
Buy someone a drink or a shot. Offer to pay for your friend's meal if you're grabbing some snacks at a pub. Hold the door for someone. Bring food or drinks to a party when it wasn't expected that you do so. Do these things occasionally as a friendly gesture to someone you already like. Don't do it as a way to buy people's affection or make them obligated to you to return a favor at a later time. If you do these things too much you can get taken for granted, taken advantage of, look like you're trying too hard to please everyone and make them like you, and put other people in an awkward situation because they feel uncomfortable taking so many free handouts.
Offer compliments to people
Don't be afraid to be positive and encouraging towards other people. If someone is good at something then tell them so. If someone looks nice, or is well dressed, then say you think so. If you think someone is funny, or a cool guy, then let them know. Again, moderation is the key. The occasional genuine compliment is way better than a constant stream of try-hard ones.
Be reasonably polite
Whatever it means to the company you find yourself with, be fairly polite to everyone. If someone does something nice, or goes out of their way for you, then thank them. Ask nicely if you're asking someone for something. Don't be an unnecessarily abrasive, self-centered, and unappreciative. Why would you do that to your friends?
Make sure everyone is having a good time when you're out
Without overdoing it and being a pest, put some energy into making sure everyone is having fun when you're out in a group. If someone seems left out of the conversation, try to maneuver it to a topic they can contribute to. Or if someone seems like they want to say something, but they can't get a word into a lively discussion, casually indicate to everyone that they want to talk. If you're doing an activity that someone doesn't seem comfortable with, try to coax them to join in (if it's harmless and you know they'll have fun once they start), or take some time to explain the basics to them if they aren't familiar with how to do it. If someone seems bored, or annoyed, see if you can get them to have fun somehow.
Spend more time with people
Spend time with people more often. Spend time with them longer. Spend time with more of them. If when you normally see your friends, you leave after a few hours, try spending all day with them. If you only see your friends once or twice a week, try seeing them three times. If you usually keep to yourself at work, and only talk to people on break, try spending time with your coworkers a little more during the workday. If you only see some acquaintances of yours under specific circumstances (e.g., in particular class, at a club), then try to see them outside of that situation. This is all assuming the people you know would be glad to spend more time with you, but if you prefer your own company like I often do, you probably underestimate the amount of time 'regular' people like to spend with each other. It can also be an interesting experience to resist your urge to go home, spend several more hours with people past your usual tolerance, and realize you actually kind of prefer it to being home alone with not enough to do.
Be interested in what other people have to say
This is one of those easier-said-than-done dispositions. Sometimes, for whatever reason, you're not in the mood and you genuinely don't care about what certain people have to say. Still, when you are interested in other people you'll naturally be more friendly towards them. One thing I consistently find is that everyone has at least something interesting about them, it may just not be readily apparent. Like you may see a guy and assume he's pretty generic, but it turns out he was a professional table tennis player for a few years, and that he's worked as a 3D artist on some major movies. You never really know about these things.
Genuinely like other people
Also easier said than done, but if you have this attitude the other points will tend to flow out of it. I find that when the rest of my life is going pretty well it's easier for me to be positive towards others. When I have specific problems weighing on my mind I'm more grumpy and standoffish, and my issues distract me from socializing. When I'm feeling socially insecure I can tend to see other people as competition or threats or reflections of my own inferiority. Insecurity can make me believe other people really don't like me, and so I 'get them before they get me.' I also find I can be down on people I'm uncomfortable with. If I can get more comfortable with them - through being around them more, or learning what makes them tick, or being able to keep up with them in the things they do - I find I'll start to like them more. Also, I find most people aren't so bad when you give them a chance and get to know them. On the other hand, you can't be expected to like everyone, and some people are just dicks.
Overall, having a friendly disposition or attitude is great if you have it, but you can't consistently create one on demand. You can keep certain actions in mind though to still be a more friendly person. Just inviting people out more, remembering to chat to people when you see them around, or joining groups, even if nothing changes about you deep down, are still the behaviors of a sociable person. You'll come across that way more, your social life will probably get a boost, and eventually, through a slightly convoluted process, your mind may come to follow your actions.