An enterprise group requested that we add the ability to automatically add events to the calendar when someone sends a calendar invite to a group. I've just pushed that to the live site, available to all premium groups.
For example, if in Google Calendar, you create an event and then add your Groups.io group as a guest, that will generate an email that's sent to the group with a calendar invite. Now, we'll parse that invite and create the event in the Groups.io calendar (assuming the sender has permission to add calendar events). The email is still sent on to the group members.
Please let me know if you have any questions.