Thanks for the responses. I know I'm thinking more into it than they did. I definitely don't want to ask them. That is good for some situations, but I don't want to create any awkwardness. I plan to be in this department (a higher level full time job, but still the same department with the same people) for at least five more years.
I can see that, if there are 10 people who work together and 3-4 people go out together, that's different. But it was just me and the new guy who weren't asked. And, I don't mean to be rude, but the new guy does not fit in to the office at all. I'm hoping he will settle in soon.
I do have social anxiety but only two of them really would have known me as the shy one (the two that have worked there for longer than me). I was comfortable by the time I got to know everyone else. I've had lots of "non work" discussions with them. Sometimes I take the bus home with them too, depending on when we leave, and we chat on the bus ride and stuff. But the girl whose birthday it was - I guess I feel "least" close with her. Maybe she doesn't like me? I'm certain most of the others do because we talk and laugh and have fun during work (it's a laid back office environment). They also know I go out and party and club and whatnot, so it's not like they think I'm someone who sits at home doing nothing, you know? I *know* we go to different places but everyone goes to different places, the city is huge, but I'd be willing to try something new perhaps. Or do something slightly out of my normal routine.
At lunch, usually everyone goes together but sometimes people split off and a group of people goes to another location for lunch (instead of the office cafeteria) and I finally, about half a year ago, started getting invites to this "other location". (It's hard to explain it without giving everything away about my life lol.) so that's part of the reason why I thought I was fitting in. Sometimes I went, sometimes I didn't, depending on how much work I had to do. As for other events outside of work - I have never been invited to one before but I haven't heard of one before (besides two of them who are pretty close, they go out together a lot). A couple of my coworkers text me sometimes too, outside of work.
Also, the girl whose birthday it is has a boyfriend, and so do about half of the coworkers (have a boyfriend or a girlfriend).
I've always separated work from friends. And I won't ever tell them about my current drug use (although I have talked to a few of them about drugs, mostly educated them, lol) and most of my "fun" involves clubbing and drugs. But now that I actually like my coworkers, I would have thought I'd be invited out.
I'm not going to let this lower my self esteem or something. I'm going to try to not let it bother me. I'm sure they didn't do it on purpose. Maybe they thought I wouldn't want to go. I'm sure some of my other coworkers would have invited me ... maybe it's just this one specific girl who doesn't like me. Could be the case. I probably won't ever get an exact answer but I do have some better ideas now.