Just relax and forget yourself and have a good time, Susan-Marie. I was invited to join in a session with a number of instrumentalists who play a heck of a lot better than I do. I was apprehensive about it, but I kinda got pushed into it because the woman who organized the session is my significant other. Well, when the music got too complicated for me to play along, I started singing along, and everybody liked it. That's the way it has worked out - sometimes I play the harmonica or guitar, and sometimes I sing. I can't say I mind it when people say something about my "beautiful voice," although I hadn't really thought of it that way myself. They don't seem to mind my guitar or harmonica, either, although I'm still pretty self-conscious about that.
It doesn't really matter if somebody hits a wrong note here or there. The idea is to make music together, and that act of creation is something wonderful and spontaneous. Just let it happen, and don't worry about it - your "instrument" can make just as much a contribution as theirs.
One thing I'd suggest - think of yourself as an organic part of the group. Don't think that they're accompanying you, or that you're singing along with them. The greatest pleasure comes when you feel at one with everyone who is involved in the process of making the music.
-Joe Offer-