An event can be associated with multiple groups. So a single event record can be displayed on the calendars of multiple groups' mini-websites.
To associate a group or multiple groups to an event, first go to the event record and click on the Groups button in the row of function buttons for that event.
You will be shown the following form to associate groups to the event. Select groups with which you want to associate this event. To select multiple groups, hold the CTRL button down (on the keyboard) as you click groups with your mouse.
Finally, click on the Associate Groups button, at the right of the form, to associate these selected groups. If any previously associated groups are not selected in the form, they will be disassociated after saving this form.