Authorize.net changes to website

I have a site owned by an organization. The members are only listed if the member is active for that year. The way the site is currently programmed: If the member logs in and pays ([login to view URL]), the database makes their membership active until the year ends and they will be displayed on the member's list. If they don't pay, their are not active in the database and their name does not display (for members that pay with cash/check the administrator has an option to update their membership manually). The problem is, member's have to login to do this. Some of the member's don't pay themselves, their office managers, secretaries or other people pay that don't have access to their login information. Instead of users having to login, I would like an option for people to choose who they want to pay for (drop down list of all active and inactive members) and be able to pay simply by choosing the name and the appropriate fields update in the database.