When you have more users than devices or users share devices and you only have Azure Active Directory the ability to switch users work a bit differently.
The first user that you enroll with will be an Administrator, all subsequent users will be a Standard users.
Microsoft Intune will block any user to enroll a multitude of devices. The limit is set in Azure Active Direcory at 20 Devices. You can change this.
To do this keep in mind that you need to be an Administrator:
Navigate to: http://manage.windowsazure.com
That is it. This user can now enroll an 1000 devices. on this domain