To restrict a marketing piece, click on the template title to get to the details page. Toggle the option for Access granted to selected groups and agents only. Click Save. No sales agents can see Restricted pieces, unless they are specifically given access.
To give a sales agent or group access, click Manage access to Groups or Manage Access to Agents. This access can also be managed by hovering 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.