Kids should be allowed to join a sport than they like. It wouldn't be fair to make a kid do another sport even if they're good at it. If they want to do a sport that they're not good at let them, they will get better and a lot stronger.
As long as the parent has the resources to pay for the activity and to get the child back and forth to it, that child should be allowed to join whatever group or activity or sport strikes his or her fancy, even if it means changing activities once in a while, for it is good to follow one's heart.
No, kids should not be permitted to join any group they want to, because there might be reasons that a child should participate in something instead of something else. A child who shows great talent for music should focus primarily on that, and do athletics as a side hobby. Also, parents cannot always afford for their children to be in any activity.