Jivtesh has a nice look at setting security for Alternate/Modified forms and reports in Dynamics GP 10. This has changed a little from earlier versions so if you are looking at an upgrade, make sure to look at this.
Access to Alternate/Modified Forms and Reports in Dynamics GP 10
Reporting & BIGP 10
Archived post from 2008. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.