Go for it. Worst that can happen is you get bored or you find out it's not useful for you.
I've met lots of AS/autistic people in real life, and they're usually fine. Some are annoying or spoiled or immature; but I've met plenty of annoying NTs too, so I don't think AS has anything to do with that! You may find friends. I have.
You'll probably meet people you wouldn't guess are autistic because they seem so socially proficient, as well as people who surprise you by just being able to be out on their own without somebody there to help them. Sometimes it's a little awkward trying to communicate with people who are just as bad at it as you, or worse; but usually they're used to that and nobody really minds. You might have to work on your own hesitance to interact with people who are obviously disabled; but that's a good thing, and you really can't accidentally insult them nearly as easily as you might think. Once it stops being awkward, you kind of forget about whose disability is invisible and whose isn't, because you'll probably have a lot in common whether it is or not. I attend a disabilities group for disability-in-general, and that's my experience--having a lot in common even with the people whose disabilities are obvious and physical, as opposed to my own more subtle cognitive/neurological/mental-illness issues.
Groups can be really fun, or really boring, or even insulting, if an NT group leader insists on trying to turn you all into good little NT imitations or something. You won't know unless you try. If it doesn't work, you don't go back; simple as that.
Regarding your not having an official diagnosis, you might introduce yourself something like, "I have a lot of autistic traits, but I haven't been able to find a specialist yet." If you have some other diagnosis, like ADHD, mention that. If the average age in the group is twenty or older, you'll probably have at least some who were diagnosed as adults, or who were misdiagnosed with something else that was later corrected. Occasionally, if the group is run by a counseling department they will want to pre-screen you to make sure the subject matter in the group is something that you'd find useful. They might want you to have a diagnosis, in that case; but that gives you the perfect opportunity to ask for an evaluation, at least one that can tell whether you have enough autistic traits to find a group useful.
So try it, see if you like it; if you don't, you've lost an hour or two of time--not a huge risk, really.