Aug 01 2019
Adding a calendar to a user's mailbox or shared mailbox and sharing to an external: No edit rights for the external user
Using an Office 365 group calendar with guests in the group: No way for guests to view the calendar
Thanks in advance!
Aug 16 2019
@JustAnotherITGuy, as it's built right now; guests to Office 365 Groups do not have access to the group mailbox nor calendar (i.e., the Exchange mailbox). They do get copies of invitations and email sent to the group.
Feel free to upvote this UserVoice item: https://office365.uservoice.com/forums/286611-office-365-groups/suggestions/17843563-provide-guest-m...