Select one or more objects. > Group menu > Attach
The Attach command makes the selected object part of an existing group.
With an object selected, choose this command, and then click a (closed) group in the scene.
Procedure
To attach an object to a group:
Select one or more objects to attach.
Choose Group > Attach.
Click any member of a group.
The selected objects become part of the group, which is now selected.