Setting a user role as either Administrator or Member will determine their level of access on the Ad Account dashboard.
Administrators have the same access as the main account holder, including the ability to manage billing and additional users on the account.
Adding a Member allows you to have greater control over access to specific Ad Accounts and to billing, as needed. A member user has full control over his account and can perform all the functions within the account. However he cannot access other Ad accounts which he does not have permission.