Being at a barmitzvah is no different than being at a big birthday party or wedding. Just be yourself. You'll do fine. Normally after the temple ceremony you go to a reception like a wedding and sit at separate tables so the crowding shouldn't be too bad. I'm sure they are having it at a place where you can take breaks and go outside to get fresh air if needed.

I didn't like crowds either. What got me through was that these people were just like me. Yes, I was anxious but anxiety never killed anyone. When you go, and that in itself is a great accomplishment, try deep breathing exercises to calm yourself. I was in the same situation and once I got inside, and started to deep breathe, I was calmed and eventually was comfortable. I had a good time but always knew that I could leave at any time I was really uncomfortable. Try the deep breathing where you breath through your nose, extending your diaphragm and exhale slowly. Good luck and try to have some fun. God bless.

I would just tell yourself these are people just like me, i do this and it does help, if someone talks to you make coversation back, after a few minutes this may help calm you.
If you have meds to calm your anxiety id take it before you go and take some with you.
Good Luck and Hugs.

Breathing excercises always help me handle crowds. And don't be afraid to 'slip away' for a few minutes, give yourself a chance to calm, then rejoin the group.

I'm sure it will all work out fine. :)