robert_e_bone
I share files with folks using Dropbox all the time, and I will almost always send an MP3 mix of the project, that the other(s) can import into whatever recording software they happen to use, then they record whatever track(s) needed, then they would export each individual track that they recorded as a Broadcast Wave file, so that it/they can be imported by me back into the original Sonar project.
 
Since I already have each track that went into my exported MP3 file, as separate tracks in my master project in Sonar, there is no need to send them anything other than something for them to listen to while recording their actual parts.  Then, when I import each broadcast wave file for each track they recorded, importing them into my project will put them in the correct location.
 
Saves a lot of bandwidth doing as above.  
 
Bob Bone