To configure which users can access to the mobile app (in case we don't want all of them to access) go to:

System --> System Configuration

On the left menu go to WEB-APP and then to the MOBILE APP Tab:

At the bottom of the page you'll see the option to limit the access of the users to the mobile app, and if you select the checkbox, more options will show.

Then we can select the options of the groups of customers we want to give access to the app, for example if it's only for customers that are paying a fee, we will select the first option.