This issue is interesting for us, too!

So the issue is that user UB can see all users in the user list, even the ones he does not own?

Though I didn't originally post this issue and hope not to interfere with riqcles - yes, this is exactly our point!
Although I admit that I cannot see at the moment how users' visibility to others could be managed to solve the issue. Mail domains are probably the only possible characteristic to filter by. But this would be only partly useful as sometimes users from different institutions or mail providers need to cooperate in a certain group or on a certain survey.

It would be marvellous if someone could come up with a good solution!

It's the purpose of "Group member can only see own group" , no ?

Except for superadmin.

As I understand it, user with the right to add or delete other users from a group should only see certain users and not all. @riqcles, am I right?

... sorry, reading the description again: I think it is "user with the right to add or delete other users from a group should only see users created by himself/herself."

Which would help us probably, too.

Yes, UB must see only user create by him (in this example UB1 UB2 and UB ! )
and UB1 can only see UB1 and not the other.

For the group, it's the same :
UB must see only UB1 and UB2