To change/update the multiseat access go to contract in question

Scroll down to included licenses and "manage seats"

There you can also see how many of the seats that are currently filled and by who. If you wish to remove a user, simply press the "x" marking by the users name

To grant another user access just add their e-mail in the top bar

If all the seats are filled and you wish to add more, go back to contract and to actions. In the drop bar choose "change quantity and price". Here you can change the amount of open slots for the multiseat