I am also looking the answers with same questions .
I would like to add here that : different category admin(special role user) should not see the users of whole site , basically when I trying to enrol the user into courses its given me whole sitewise user list to choose , which is serious privacy threat for other categories(organizations) users.
what I have done :
I created a special role and given all the required permission according to categories. then I given that role to some user (category admin) .
Site admin --> Users --> Define Role .