If I'm not mistaken how I understand it right now, the only possible way to get interest groups working is by
using (construct) events to loop through a list of ID's (regions) that are in the interest area of the player
and use the Raise event action to send data for each ID (region) individually,
this would render the Change groups action useless though.
"Change groups" subscribes client to and unsubscribes from events sent to specified groups. Client can listen to multiple groups. But event can be sent to single group only. So technically, to send to multiple groups, you need to create new group containing clients from all groups you want to send to (subscribe all these client to this supergroup). I'm not sure that this is realistic solution, especially considering number of groups limited to 255.
Note that mmo concept you mentioned is implemented with server support and does not rely on groups.