1. Select contacts
   
   
2. Show the group contacts by clicking the Group Contacts icon
   
   
3. Click the + button in line of the T button so selected contacts will be added to that group
   
   
   
   
4. Automatically, the selected contacts will each have that on-the-fly group checkmarked.
   
   

Note: To remove (not delete) the contacts from the group, just click the - button or the C button to clear all contacts that has checkmarked from that particular group.