Hi Umberto,

There is no easy way to do this at present, there is some guidance to possible workarounts as below:

https://moodle.org/mod/forum/discuss.php?d=205027 

http://tracker.moodle.org/browse/MDL-33823 

"Basically in short moodle/course:view is an admin capability that does not hide courses but allows site administrators to view the content of courses without being enrolled. By design, all courses are supposed to be visible to all learners and there is currently no way to hide courses by role in the way you would like. There is still ongoing discussion on how best to allow more flexibility on course visibility, so things may change in future. 

Now in saying that, there are a couple of possible workarounds, coming at the problem from two different directions. 

Firstly if you want to hide courses from most users and only show them to users in a particular role (I realise it's the opposite to what you requested but still may be useful information) then you can set the course to hidden (course settings, Availability, select "This course is not available to students"), and then in the Course Roles, Override Permissions tab, override the role you want to see the course, setting capability moodle/course:viewhiddencourses to Allow. 

Alternatively, to hide particular courses from a subset of users who are assigned to a particular role, then the only way to do this would be to hide *all* courses by default, give the site Learner Role the moodle/course:viewhiddencourses capability at site level, make sure the capability in the Agent role is Not Set, then override course role permissions on the subset of courses that the Agent role is allowed to see, giving them the moodle/course:viewhiddencourses capability in the course context. 

You would probably also want to remove/change the CSS that shows hidden courses links with a dimmed gray text on a site like this. They are found in theme/standard.styles_color.css

I realise it is a pretty clunky workaround but there doesn't seem to be any other way to do this with the permissions system as it stands in Moodle 1.9/Totara 1.1"

There is progress being made on this problem, but there is no solution date as yet.

hope this helps,

Regards,

George.