I understand that you create these groups to keep chats private.
I can think of possible plugins:
Inform in newsfeed that a hidden group has had activity (without saying what activity) and recommends requesting admission to the owner
Prevent groups with less than x members or prevent anything from being published until they have enough members, encouraging them to invite people
Self-eliminate groups that do not have enough activity and members after a certain time
Limit the number of groups a member can create based on their user role