Youth groups are the same the world over, whether religious or not. Basically young people are clique-y, and no one will make the first move to welcome a newcomer, they'd rather wait and see what their friends think of them first and then follow the crowd. Go back again and find some common ground (a shared taste in music or opinion on something etc) but don't try too hard - remember the new person is very interesting to them, they will want to know what you are like. Just be yourself and give it time.