Could you please explain what you want to achieve?
If this is homework: there is a
special forum for that. Please repost there and get a moderator to close this thread.
Otherwise: "read-write" or "read-only" are not attributes of a user but only of a user-file (or user-directory) relationship. This means, UserA can have read-write privileges for FileA, but read-only privileges for FileB, while for UserB it the other way round: RO for FileA and RW for FileB.
What "Admin user" should mean is completely unclear. Such a type of user does not exist in Unix. There is only one special user, which is "root" - to be precise any user with userID 0, regardless of the username. Because userIDs are supposed to be unique this means effectively that you can rename "root" (very bad idea usually), but not more.
I hope this helps.
bakunin