When you sign into your Glean Admin account you'll see an overview of your account status. This page will show you how many Invites you have remaining and your maximum user amount. In the screenshot below, the account has 15 users total, 7 invites are remaining, meaning 8 are in use.
You can return to this page from the User List by clicking the Glean logo in the top left.
From this page you can: