Admin users are user which have been granted access to the Admin app. Non-admin users do not have access to the Admin app.
By default the first innius user in a company is an admin user, and from then on new users are non-admin users.
Other company users can become admin users when an existing admin user grants them access. To do this open the company page in the Admin app (1).
Then open the Administrators panel to manage your company's admin users. In the top right of the panel click the more option and the button 'Add' (2).
Now choose one of your company's users to become an admin user (3).
If you need to remove a user's admin rights, this can be done by clicking on the delete button (4). Note that this only removes a user's admin rights, making them a normal non-admin user.