How can I access Office 365 (Private) Groups using Apple Mail?
I have not found a way for the IMAP sync to work. My staff are using either the teams iOS app or Outlook iOS app to get at those group assets like files and mails. It’s not the same experience as using Apple’s apps and it is something you’ll want to use Microsoft user voice or your company’s  Microsoft technical account manager to advocate that they enable normal IMAP sync for those folders.