Seats refer to the licenses that you pay for you and your team to access Goodshuffle Pro. In order to add a teammate to a "Seat" you must invite them.
From your Account Settings, you may view your seats under "Subscriptions".
Find the empty seat, input your teammate's email, and select. "Send Invite".
Have your teammate keep an eye out for an email with their Company Invitation. (If there are filters on their email inbox, they may need to check their Spam)