I have dealt with the same issue. Wanting my wife to have the same access to my almost 4K songs without her having full access to my main Google account. Remember, Sharing your google password is a NO NO. Even with a family member. It give access to more than you can imagine. Web history, every photo you've taken, downloaded, or deleted. Web history, location history, and text, call, ability to track you via GPS, remote everything, and or wipe, lock, or completely erase.... and thats just your phone. Computer, Tablet, even track what you were doing on a friends computer even if your on the other side of the earth. If your logged in, more data is being collected than you think. Even if you think you know. And its all accessible. By you, Google, anyone who has your email address and password, and now (since a year ago bill passed) the Government. The ability to secretly track, take pics, video, and record audio, when your phone looks like its off. Unless your batteries off, its possible. Just knowing your email address, begins an array of info linking to info on you personally. Ever googled your email addresses? Scary. So with that being said, you shouldn't just log in with here phone, giving her access to your music because you cant separate everything else listed above. Its so scary, I dont even want to know my own Google Password. LOL. The only ways to share your music library is to...
A... Download your entire music library, log out, log in to her google music, then upload that folder. Note: It will be a large file. Even Zipped. (Ext. HD suggested) AND you can only download your music twice. EVER. one song or everything.
B... Upload your Music folder (once downloaded) to your Google Drive. Thats a Google Feature that you CAN SHARE with other Google accounts. Any type of files. Pictures, music, documents, or extensive that only special programs can open. From there, she could pick songs and play them one at a time, or down load them to her phone, or DL/UL to her Play music.
C..... I really like the shared Google account option. DL/Up load your music to a NEW Google account and share everything from Music, Books, and Emails. You can even pick what you want to share (sync) on your phone with that account, Giving access to Photos, and what ever. Just remember not to use the same password for the new account. (That make the whole point useless)
D..... What I have done so far is, download or buy music online with my computer, and upload it all to MY play music, KEEPING the music files on my Hard drive so, if she wants any music I have, I just log-in to her account, and upload it from the same source I uploaded to my account with.
F it..... I'm taking my music, and uploading it to my own server.Then I can do what I want with my music. So, neaaaa (Sticks tongue out)
It does stink that you cant simply share music like you can contacts, pics, or documents. Before the internet was dominated with massive servers, the web was mainly peer to peer. So with that being said, I support Torrent client software, and the index search engines that bring us all together. Long Live pirate Bay and its developers. (whom sit behind bars because other people share stuff with one another online) And these guys just made finding things to share simple, without uploading or sharing anything copyrighted themselves. Google Pirate bay justice for more info. You cant write your name on everything. I paid, Its mine, and I like to share. As long as you "Be Fair, when you Share"