To restrict a marketing piece, click on the template title to get to the details page. Check the box marked Restricted. Click Save. No sales agents can see Restricted pieces, unless they are specifically given access.
To give a sales agent or group access, hover over 'Marketing Pieces' >> 'Manage Restricted Access' >> 'Manage Group/Sales Agent Access'.
From here you can locate the restricted template, and drag over any agents or groups that you want to have access.