From your dashboard, select Plan and billing. Then on the left-hand side, select Users. Here's where you can see how many users your account currently has.
To invite more users, navigate to the top left corner and click on Invite user. Then add the email of the user you wish to invite and click Save changes.
Alternatively, you can also reach the Users tab through your dashboard. On the left-hand side, select Plan and billing. Then under Usage, you can see the number of user seats. Click Manage users to be redirected to the Users tab.