From the Manage Users page you can restrict your Trainers', Managers', Team Leads', and Reporters' view. (Administrators cannot be restricted.)
Locate the user and select their name. Click on the Manage button that appears.
In the Overview tab, click on Edit under the Permissions section. Select the radio button next to "The following groups" and then under the drop down menu choose the Groups they should have access to. Hit Save.
When they log in, they will only be able to see Trainees who are a part of the Groups you selected above and all reports will automatically be filtered by these Groups, too.