The email address that you originally sign up with (whether for a new company account or when joining an existing company) is the email address you will need to use each time you login to Built for Teams. To change the email address or the password used for login, follow these steps:
Login using the original email address.
Click the gray arrow at the top right of the screen, by your profile photo.
Click on "Account Info".
Change the email address and/or password, and select the Update button.
The next time you login, you'll be required to use the new email address and password.