From the Partner Panel it is possible for a partner to add a user to a customer's account.
Alternatively, the customer can add users to his own account by following these instructions: Create a new user
To add a user to a customer account:
1. Select the lock icon near Tokens under Details.
2. Select Create user.
3. Select a role. For a list of available user roles and their permissions, see: Available user roles
4. Enter a name and email.
5. Create and confirm a password.
6. Optional. Set an expiration time.
7. Select Create user.