You will need to be on a Growth plan or higher and you can manage your users and their seat type(s) from two areas.
User Management
From the left hand menu, click Company Information, click User Management. This will take you to the user management page.
Located at the top of the screen is an option you can +add seats.
A popup box will display where you can add or remove seats from your plan.
Or you can manually update an existing user’s role.
Your account will be updated automatically.
Billing & Subscriptions
From the left hand menu, click Billing & Subscription, click Plan Overview. This will take you to a summary page.
Under the Current Users you can select manage users.
A popup box will display where you can add or remove seats from your plan.
Your account will be updated automatically.
You can view the number of seats available from the user management tab, update an existing user or invite a new team member to join your workspace.