Converting GarageBand audiobook to MP3 for non-Apple users and resolving sharing issue on Mac

I created a GarageBand audiobook on my iPad. I’ve sent it to a small group of people. Some of them can’t open it, and I assumed they didn’t have Apple products (so couldn’t get GarageBand) & that I’d have to convert the files to MP3 files. I’ve gone to my Mac to see if I can do it there, and while I can see the folder, and the separate chapters/files within the folder, I can’t access the sharing/convert to MP3 as it says there is no current project ‘open’.

1) So first, I’m wondering how to get access to the folder/files on my Mac so I can convert them so non Apple users will be able to listen. (and I’m assuming that is the thing to do)

2) But in the meantime, a family member who I sent a test email (it worked for me & partner with iPads & GarageBand loaded) said he couldn’t access it, sending me a message that says “Item Unavailable: The Owner stopped sharing, or you don’t have permission to open it”. Under sharing, it still says this person was sent an invitation. Though I had changed one setting - I changed it to ‘view only’ rather than ‘can make changes’, since this was a completed recording I was sharing with others to simply consume for our book club. [in less than 2 weeks now. Help! and Thanks!

[Re-Titled by Moderator]