By age 11, they should have done the 'pants rules' at school, which makes it clear that it is your body, and you shouldn't be pressured into showing anyone else/letting anyone else touch you etc. If the other girl felt pressured into it, that is really unpleasant, and it's sad that she was upset by it.
OP I agree with others that this would be perfectly normal at 5 or 6, but at 11 it isn't. We're one of those laid back houses who are happy to wander around naked, and DD doesn't think twice about coming in to chat to me while I'm in the bath, but she's been very keen on her own privacy, and keeping covered up, since she was about 9.
I agree, therefore, that you should have a gentle chat with your DD about respecting other people's desire to keep their bodies to themselves. On another note, I know that this kind of behaviour, at this sort of age, is often a flag with Social Services who are already keeping an eye on a family, as they consider it a sign that a child may be encouraged to play these kind of games by an adult, and may be being groomed. I am not saying that's happening with your DD at all, but when you have a chat it would be good to keep it in mind, so that you can pick up on anything she says that might indicate that.
It might just be that this is just your DD's character, and it's most likely that nothing was meant by it, but she does at least need to know that it's not really on. It reminds me of those communal changing rooms in the 80s where some people would completely strip off, and others would hide under their coat! I was one of the strippers 