In the online versions of TurboTax, you can only file one tax return per account.  So if you want to prepare someone else's return after doing your own, you'll need to create an account for them with a username & password (you can use your same email, if you like).

Once you sign out of TurboTax, you can hit the "sign in" button and when the sign-in page appears, look below the username & password box and click on the create an account link.  That will get you started.