You can access this window through the menu Vehicles -> Manage groups of vehicles.
 
This window allows you to create and edit groups of vehicles.
This is the list of all existing groups.
This is the list of vehicles of the selected group.
To add new groups through the Groups of vehicles add/edit window.
Delete the selected record(s).
Edit the selected record.
To add a vehicle to the selected group.
To remove vehicle(s) from the selected group.
Close this window.