Definitely the more you perform the easier it gets. In the interim, make anyone and everyone listen to you. Practice the introductions along with the song. The more you do it, the more confident you'll feel for the real thing. Never mind if you are on your own singing only your part (even if it makes no sense alone), just get people to listen.
The more nervous people are, the less they tend to play/ sing in front of others, and that makes it worse when they actually have to. Sing to your family, your friends, as much as you can.
I've had some very nervous pupils and I usually gather them together a few times before festivals and exams to play to each other. Initially they hate it - but it does get easier and make the real thing better.
Also, try to think of the fun it will be and the entertainment you will give, regardless of what the other groups are like. They could be world famous professional musicians or they could be singing for the first time in their lives, and you would be just as good either way. It is easy to get fazed if you know other people are good, but it's a red herring, really. And remember, no one is unbeatable. They may be brilliant, but they may not have come up against you yet :)
Glad to be of any help I can. Do try to relax and enjoy it because these things can be great fun, and as you say, being relaxed is key :)