Actually, the topic says it all 
I've searched my ass off to discover why a shared notebook wasn't visible, only to discover Joplin Mobile doesnt show the yellow bar on top like the desktop app does, asking you to accept/deny a request from someone who wants to share his/her notebook with you 
I think Joplin is A-MA-ZING! But maybe this could make it perfect?
I have the same situation. I wandered why I don't see the invitation. Then I realised that I have to install the desktop app just to accept the invitation. It would be great if this was improved!
Especially in situations like mine, when you simply don't use the desktop app.
I also noticed the other limitation: When you use the encryption, new shared notebooks appear on the desktop app without a problem. But on the mobile app they appear as "
Encrypted".
To workaround this you need to disable encryption on the phone, reenable it, and wait a little. And also relaunch the application to get rid of yellow banner that shows you that you should enter the password.