If you go to System Administration > User Administration you will find all registered users. Here you can add new users, modify existing users and delete users. If you want to change the plan of a user, click the pencil button and a new page will open where you can select a different plan. Also you can update the expiration date and set an account active or inactive. When an account is inactive, the user can't login.
There's the User Administration (System Administration > User Administration):
Owners have access to the complete system. They're the root users with access to system administration like user accounts and plans.
Admins are regular users. They own an account and can create apps. They're called admins because they're the admin of their own account. They can manage team members for their account.
And there's Team management (Settings > Team):
Managers are team members within an account. They have the same permissions as admins, except for team user management and logs. They can create and delete apps, but they cannot manage user permissions on these apps.
Users are also team members and can only edit apps they have permission to. They cannot create, clone or delete apps.