Let’s scope this down a bit.
Keep the current UI as is, only change if there are more than 50 members the group members UI becomes readonly.
I think it makes more sense to keep the UI for adding and removing users for large groups under
/admin/users/list/active?group_id=99
This way we look at different orders later on. search for a user in a group. and reuse the existing UI.
For example, the new UI will not allow you to easily search by email for a user in a group, will not show all the stats and so on.
If you want to get started, I say get started on #1, totally fine with those changes to the admin/user page. Please account for groups that you can not remove (like trust level and so on, don’t show an x on them)