Hm, this depends on how it's moderated too. Maybe you could talk to the person who leads the group privately and ask if they could make time for some more intros or introduction/getting-to-know-you exercises or exercises where people get mixed up and talk to others and walk about the room? Also tell them you're shy and that sometimes you do have things to say but may need some encouragement or structured space - or whatever would be your preferred way of working?
(I've never been to group therapy but it can't be that different from workshops and classes?)
I've felt isolated in some groups before too, especially in primary school or high school, when I didn't know how to talk to people etc. And if there were no structured ways to talk to people..
I agree it's best to find some friendly people or some who share your view on certain things, and talk to them a bit before and/or after.. And smile and greet everyone when you come, look them in the eyes.. Ask them about certain things you liked or found interesting, or would like to know more about..?
I fret before joining a new group too, though mostly it goes quite cool and I'm muchly relieved afterwards.. could you just take it as a 'challenge' and expect to be mightily relieved afterwards, and reward yourself with something nice?
