Reaper is awesome, but there are a bunch of different ways to do the same thing. I'm glad I saw this post because I haven't really used the folders much, now I want to go in and figure them out so I can see if they'll save me some time. Oh, and the manual is one of the best I've ever used.
Opren manual... go to folder tracks. (it has pictures that explain it better than words)
In Reaper a track can be a bus and setting it to be a folder groups all tracks in it. Simple and elegant.