Stefano,
Ah, yes. In that case, Shirley is right: a cohort is the best way.
The issue will be that you need to make sure to add each new user you add to the Cohort when you create the user. That may be easy depending on how do that on site. (If not, one way to get people into cohorts automatically is use one of the plugins that add users to cohorts based on their profile fields, such as https://moodle.org/plugins/local_profilecohort. )
Ah, yes. In that case, Shirley is right: a cohort is the best way.
The issue will be that you need to make sure to add each new user you add to the Cohort when you create the user. That may be easy depending on how do that on site. (If not, one way to get people into cohorts automatically is use one of the plugins that add users to cohorts based on their profile fields, such as https://moodle.org/plugins/local_profilecohort. )