To access the self-service options go to account.com.  With already designated seats that you have previously purchased you can then assign an employee (user) to a seat. Here's how you do that: 

1. From the User Management menu, click on the user you would like to assign a seat to.

2. Next click the EDIT button.

3. Navigate to the Seat Assignments section and check the box for the subscription you are giving the user access to.

4, Scroll to the top and click SAVE.