After a campaign has been run you can filter results and see who and who didn't engage. These filtered results can be assigned to a group. 

Assigning a group doesn't over write the existing group it simply adds another group name to the contact.

This is really useful if you want to target a certain set of people that have responded (or not responded at all) with a followup campaign.

To assign a contact to a new group look for the 'Add these to a Group' button at the bottom of every report or dig-down. 

You'll need to type 3 or more characters before you can the filtered result .