"Can I buy sheet music for a solo and take that to make my own accompaniment using Finale?"

As long as you've purchased the music and you're using it for your own (private) purposes, this should be fine.

"Then can I take that and share that accompaniment file with others who have purchased the sheet music as well?"

I don't think this is a problem either, as long as you are privately sharing your work among friends, who also own the sheet music. Who knows if the copyright holders would agree with me, but it seems reasonable - you're just doing work for friends that could do it themselves since they have the sheet music.

The problem comes if you post these files PUBLICLY for sharing. You might then need the same kind of multimedia license that MakeMusic has for all of our SM accompaniments, and would be required to pay royalties on it. I have no idea how publishers would respond to a request from an individual, but it's certainly worth a stamp to find out.

The Finale Showcase is strictly original works or Public Domain. On the Finale Showcase, if we see a copyrighted piece of music posted we remove it as an infringement of copyright.

If you have requests for songs to add to the SmartMusic accompaniments, please just drop us a note at smartmusic@makemusic.com.

Hope this helps!