Pranas,
You can restrict users to Domains and Groups from within the MRE User Administration.
You will probably want to create Reporting Objects within said domains - that way only users with access to the domain can use the Reporting Object to create their own Ad-Hoc reports.
Additionally:
Users can have differnt types of access inside a domain (i.e. HTML user (run only), ANALYTICAL user (ad-hoc), Schedule, Advanced, etc.
There are also Groups within User Administration, so a user can have access to one or many Domains, and also be part of one or many Groups, thus allowing even more flexibility.
Metadata Security. While this might appear to add a LOT of maintenance, this is typically not the case. Let's say you have two users running an HR Salary Report. One user is new, the other is "senior". You don't want the "new" employee to see salaries over a certain amount. The "senior" employee can see everyone's salaries.
You can write just one report to satisfy both needs by using DBA security in the metadata by creating an exception rule for the new employee (i.e. where user "new" SALARY LE XXXXX).