In Outlook 2016 for Mac, select the Groups node to expand it. Your top10 groups are displayed.
Select the desired group.
Browse the group conversations until you find the event you want to add to your calendar.
Select Add to my calendar.
Note: It's not possible right now to view the group calendar in Outlook 2016 for Mac, but you can schedule a meeting with the group by adding the group name to the To line of your meeting request. See Schedule a meeting on a group calendar for instructions.