Someone can steal a group?
When they do, it's usually because they got the user/password from the manager in some way. Usually by fairly basic subterfuge rather than sophisticated "hacking"
If you're not the type to answer emails like "Hi, I'm from Yahoo, we need to know your password because we're doing an audit" or you don't have a password of "bob" then the chances are it'll be fine.
But, to be complete.....you can never completely dismiss the possibility that a bug in their software could lose it. But it's pedantic, hypothetical and applies to every piece of computer software and every site. There's also a kind of safety in numbers thing I guess...if you knew how to steal a group because of a bug, which of the few million would you pick before the bug was discovered and fixed? It would probably be one they are already in, or one that they fell out with the manager of, or a huge popular one etc rather than a random group with piano teachers in it.