They have the concept of "shielded" users, which are users that are shielded by the shared secret, but don't contribute to it. Though, they don't really specify

They have the concept of "shielded" users, which are users that are shielded by the shared secret, but don't contribute to it. Though, they don't really specify when to put people into the shared-secret group and when to leave them as shielded. I suppose it's an exercise for the user.