Group admins use the Events tab to add events feeds from internal or external iCal files. This format

“…is specified in a proposed Internet standard (RFC 5545) for calendar data exchange. The standard and file type are sometimes referred to as “iCal”, which was the name of the Apple Inc. calendar program until 2012 (see iCal ), which provides one of the implementations of the standard.”

“To import an iCal file, you need to click the arrow button next to “Other calendars,” then click the “Import” option. A window will pop up asking which file you’d like to import, and which calendar you’d like to add the file’s appointments to. If you want the events added to their own calendar, create a new calendar before importing.”

From Wikipedia.

Group admins can also import one or more events from an *.ics file.

All signed-in members of a group can manually add an event, but that is done outside the Manage option.