To add a new user as an agency, the invitation process is used. A user with Billling Admin permissions will need to go to their "People>Invitations" file path.
This will open a screen (Manage Invitations) that shows all of the current invitations that are pending for the agency, and two buttons available to create new invitations.
The one on the left, "Invite Someone" opens the single invitation screen.
Enter their first name, last name, email address, and confirm the email. The provider will default to Billing Administrator's agency, and the user type dropdown offers two classes of user, "Office" or "Service Provider". Select the appropriate user type for your invitation, and then click "Send Invite"
The agency end of user creation is at this point, completed. The end user whom the invitation was sent to will then need to follow the link that they receive to verify their identity and link their profile to the agency that sent the invitation.