Is it possible to share files between iCloud Music Libraries?
I understand the issue and simply, the answer is no.

Apple's Family Sharing is limited to iTunes purchases (music, movies, TV, books). What it does not provide is access to the main account's full iCloud Music Library. While the main account can add, access, and delete non-iTunes media to the iCloud Music Library, it cannot share that content, even with those in Family Sharing. I think it stinks and cripples the value this service. 

I can only imagine this is Apple (and thusly, music industry) trying to thwart major music peer-sharing in constantly shared "Family" libraries.