If I file or pay in TNTAP on someone else's behalf, do I have to create a username and password for each client?

No, TNTAP allows you to setup one username and password to manage multiple tax and bank accounts.

You can save bank accounts by tax account, company or use one bank account for all tax accounts.

You may choose to have separate TNTAP logons for your personal tax accounts and your client accounts. However, you can manage both from the same logon.