When I'm 1 on 1 with a girl I do really well, I'm comfortable with the girl but once the group gets bigger and some other Alphas come along I just kind of hole up and not really attribute much after that.

Such as today, I got invited to a grill out at a park, I showed up and chilled tossing a football and a cute girl who was invited came and sat down. I thought she was around a HB6 just on looks, and when we sat down and it was me on her side and a mutual friend on the other I was really confident talking to her and opening her, really got her talking and exchanging stories about traveling, she was laughing, etc., but once a few more friends came and they all started talking to her as well I kind of just shelled up. After awhile I even recognized it but didn't know what to do to fix the situation, I tried adding to conversation but to not much avail. I just didn't know how to put myself on the same level as the other guys (one really takes the spotlight in group settings, making sure he is involved and in charge of conversation) because my jokes weren't rolling as well or my comments wouldn't keep conversation going, but I could do it just fine when I was again 1 on 1 with her.

TL;DR How do I keep all the confidence I have being 1 on 1 with a girl going into group settings?