Hi Kirandeep,
To apply Policy on particular user group follow below procedure with Snapshots:
Create New User Group under :
Add new group and select particular usaers from AD, on which you want to apply Policy.
After creating group, go to Policy-> Add new Policy and go to Group tab in Policy:
Click on group and Add Rule to select that group.
Selcet Sender/User Matches based on directory server from: Select Group of those particluar Users.
After selecting group then you can Add detection rules on those users:
Hope this snapshots will help you to create Policy on Particular users.
Regards
Axay Cumar