An error occurred while saving the commentDavid Beaumier commented
This is a really annoying issue for our users. I've even been told by some users that they won't use the group calendar until this gets fixed :-(
Adding the checkbox to Outlook as suggested would help, but why not add a setting at the group level to default this value? This way, each group could decide which behavior is best for them.
This related suggestion should be fixed too: https://office365.uservoice.com/forums/286611-office-365-groups/suggestions/31405348-allow-group-calendar-appointments-and-including-a