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.