I want to see the list of subscribers to a group that I created for the same reason I want to see who is following me on Mastodon: 1) so I have some idea who the people are who are interested in what this group produces, and I can steer content in the direction that’s actually appreciated by the subscribers and 2) so I can invite people who I think should be in it, but aren’t yet, and 3) so I can (potentially) preemptively block.
Re your point about privacy: I would think that if I create a group, and moderate it, I am the host of that group. I decide who gets to be in and who doesn’t, just like IRL who gets to sit on my porch and who doesn’t. Privacy does not really enter the picture here in my view.
This does not appear to be an applicable analogy here. If it were, Mom (presumably the owner of the house) would not be able to identify anybody in the house, only how many people there are in total in the house. Not something she’d be happy with, I assume, nor you. (Strangers? Friends? Family?) And your complaint appears to be that you didn’t have ownership rights in your room – but if you had had them, according to your version of privacy, you could only tell that there was one other person in the room with you, but not that it was Mabel. It could have been her dad. Presumably also not what you would have wanted.