New objects are added to previously created selection groups with the Add To Selection Group command.
Location of Add To Selection Group Command
After selecting the object, you can access the Add To Selection Group command from the menu that appears when the right mouse button is clicked .
Select the objects to add to the group.
From the right-click menu, move your mouse to the Add to Selection Group line.
Click on the group you want to add from the list.
The selected objects will be added to the group.
Selecting objects to add to the group
Selecting the group to add the objects to and adding the objects to the group