It's always been possible to have events in more than one group. But the field on the event form was always a mess, and confusing. Finally we've had an idea and some time to make it simpler.
On the form when you create an event, or edit it, all it says (under the location of the event) is the groups it is, or will be in.
On the page of an event that you can edit you will now see a link to add (and remove) groups as available.
If you click the link you will get a form offering you groups you can add to. Yours, and others that you can post to.