[Geoff Bacon]

If the songs are in files on a different computer, how about renaming the files to show the group and category?

e.g.
Copy all "songs" for group A into a new SongsA folder
Copy all "tunes" for group A into a new TunesA folder
Copy all "harmonys" for group A into a new HarmonysA folder
Note: using copy rather than working on your original files

Rename all files in songsA to include -AS as a postfix to the name  e.g.ABC.pdf becomes ABC-AS.pdf or ABC-A-S.pdf
For TunesA, use -AT and for Harmonies use -AH
All files now have unique names
Import each folder (putting it into the correct collection as part of the import)

Repeat for group B songs using BS, BT and BH

There are probably other ways of doing it but this means that all files end up with unique names, are in the correct collections and you have a single library.

Google "rename files in directory postfix prefix" for ways to rename with a postfix
One advantage of this method is that you do all the major work without involving MobileSheets - i.e. you won't muck up anything already in MS.

Cheers
Geoff