GarageBand: File sharing and adding tracks

My apologies if this has been asked as nauseum: I'm getting the piece that lets me connect to my MacBook Pro, which I have been trying to do forever. I think of GarageBand as a musician's "sketch pad": it can do some stuff, but not "radio quality", that's what Logic is for. Is it possible to receive a file from someone, add a track or two, and send it back? Hopefully it syncs up so they just drop my track(s) in to their project seamlessly.

And I'm guessing that this is easier in Logic, which will have to wait for now!

I'm probably getting a little ahead of myself and the answer is in the manual; it's just not popping out at me this second!

Thanks for your help and patience!