Hi,

Currently we don't have an interface to do this but technically you can do it through database.

If you are comfortable with doing it through database, then I can guide you through on the steps to achieve this.

1. Go to backend -> Events
2. Retrieve the ID of the event that you like to change. Screenshot: http://screen.stackideas.com/2014-11-11_1753.png
3. Retrieve the ID of the group that you like to change in the similar fashion at backend -> Groups
4. Execute the SQL:

UPDATE `prefix_social_events_meta` SET `group_id` = '$groupid' WHERE `cluster_id` = '$eventid'

* replace prefix with your table prefix
* replace $groupid with the group id retrieved in #3
* replace $eventid with the event id retrieved in #2.