How many users can I add to my Apple Family Sharing account?
My family sharing account is full and I can add my brother. Why?
Question marked as
No such thing as a Family sharing account.
You can create a Family Sharing group, with up to 6 members, 5 + the Family Organizer, but each member keeps their own Apple ID account. Accounts are not shared at all.
If you already have 6 members (Apple IDs), that you cannot add any more.
If you are referring to something else, then please clarify.