Open the R-groups Manager.
Locate the entry for the desired R-group in the left-hand sidebar list.
Click and drag that entry up or down the list, and drop it onto the listing for the R-group you want to merge it with.
Click the Merge button in the confirmation dialog.
The R-group label for the entry you dragged and dropped will be replaced with the label for the target entry in all instances. All descriptions will be moved to the target R-group and listed as such in the application.