it's that you want ?

Just select something
go to the Groups wanted
Click on it "Assign Selection here" = Selection go to this Group!



PS Rename as you want if necessary because Group01, Group02 are not very descriptive! :)

If you want only one group from the previous...so like a "merge"...
Ungroup the Group02 selected
as it's yet selected...Goto Group01
Click on it "Assign Selection here" = Selection go to this Group!
All is inside one group...



Like this you can make some complex arborescences without big headaches! :)