You can add or edit the staff username, staff name in "Settings" -> "Staff management".
Staff has two roles: admin or manager. Admin user can manage other staff. Manager does not have the permission to view and manage other staffs.
A staff has permission to certain topics. You can segment your staffs by their job assignments based on topic.
Note depending on your paid plan, the staff/user limit is different. You can always add staff beyond your plan limit but there will be extra monthly fee for the additional user.