Enable Live Accounts
Currently you cannot login with you LiveID to the app. Very often in collaboration with partners they have no business account and therefore they cannot use the app to access o365 conntent
James Jones commented
This should be expanded to all guest accounts, not just live. The limit for work/school accounts has been removed from SharePoint Online, but not from the app. This is silly and frustrating. Please allow personal accounts to login to SharePoint mobile app.