I posted a response to this and then went away and thought about it some more. Please bear with me as I ramble a little about fitting in with a group. This is my experience, I don't know whether it will resonate with you.
Groups are hard for me. Always have been. Part of it is temperament, part is lack of confidence, and part is how easily it reminds me of being bullied in school. Parties where more extroverted people are having fun and seem completely un-self-conscious are tests of endurance for me. I've done a lot of cooking for crowds and for years while living in a rural community I used to be the person to call to take care of the food if anyone wanted to have a dance or other event. Sometimes people would come and join me in the kitchen, wanting to give me a break, feeling sorry for me for being in there missing all the fun. They were missing the point: having a clear role to play (serving food, but it could have been anything) was the only way I could be there at all. Take that away and I wouldn't be enjoying myself, I'd be panicking and wanting to go home. It's been six or seven years since I've tried to be part of a crowd like that at all.
Finding my place in any group situation is uncomfortable and to be honest I do a lot of avoiding. When entering into a situation where other people already know each other, it can be very uncomfortable unless I know exactly what I am expected to do. I feel like I am getting in the way and not sure whether I am wanted. Just today I was in a situation that replayed that dynamic, and it was very subtle, but soon I was having trouble catching my breath, which is one of my signs of anxiety getting out of hand, feeling I stuck out like a sore thumb, and really really wanting to go home.
This forum is a pretty safe space, but I wonder whether there is some aspect of it being a group situation where you are relatively new that is making you hold back, not sure where you fit in or whether you are wanted. (You are - but maybe it's taking a while to convince yourself?) Does this come up in other group situations, or just here? Just a thought.