I can understand being uncomfortable around new people. I'm much the same way. I'm usually very introverted around new people, especially if there is nobody at all that I already know that I can "cling" onto.
Once I get to know new people a bit, I probably annoy the hell out of them, by gabbing their ear off, and making my corny puns and stuff, especially if I've been drinking (which is pretty rare nowadays). The only way I might immediately open up to someone new, is if they are super outgoing, in a non-overpowering way.
All that said, I agree with the other opinions here. If this party is going to be at Birthday Boy's home, the place is likely relatively autistic safe. Especially if some of BB's other autistic freinds have been there before. I would think BB's parents would have taken alot of the different behavioral characteristics into consideration, and taken measures to to make the home as safe as possible for them.
This would also be a great opportunity to possibly start a new network of friends with autistic kids, and be able to share experiences, and offer each other advice on issues related to having an autistic kid.
Plus, I'm sure it would be good socialization experience for the kids, since, as I understand it, socializing is one of the issues many autistic people have difficulty.