You'll require licences for any users who need Mobile access. You can manage your paid licences on the Billing page of the Dashboard.
Licences can then be allocated to your users by assigning them the 'Mobile' access within the Users page.
If you don't have any available licences, you will be unable to assign 'Mobile' access to the user.
In this case, you have two options:
1. Add More Licences on the Billing page. This is the best option for users who need regular access to the Mobile App.
2. Reallocate a Licence from an existing Mobile user on the Users page. This is a good option for infrequent or ad-hoc users of the Mobile App.