I know what you mean. I'm an extreme introvert and I shut down in big groups. A group of 3 is actually perfect for me - enough variety so I can jump into a conversation and check out at will, but not big enough that I get lost.
But believe me, one big personality per group is quite enough!

Nobody
needs you to be big and entertaining and to "win them over." You don't owe anyone that. That just reeks of desperation if you aren't naturally a big personality, and then if you leave the group yourself because you're self-conscious, you're missing out on some awesome stuff. Just present yourself as you are, and let them decide.
The world is big enough for everybody, and that's especially true for artists. Just because another guy in the group is a photographer, doesn't mean you don't have anything to contribute. Despite the stereotype of the lonely starving artist, bouncing ideas off other artists is how most great art happened back in the day, and still how it happens now. Sure there will be some people who prefer something new (or someone new, it sounds like what you're afraid of), but there are always surprises with people. Everyone can contribute.
Who knows, if this guy is a good photographer, he can be a mentor to you or give you connections to jobs. That's how I got a pretty sweet freelance gig. Networking is the name of the game nowadays. The client didn't even ask to see my portfolio.

If I'd closed myself off from other artists thinking I wasn't good enough, I would have missed out on that.
You have to believe there's room for you too.
Once you believe that, all these opportunities will start to open up for you, that you didn't realize you were closing yourself off to in the first place.