If you're handy with the REST API, there's actually a call that should be able to help you with this. There's a "participants/recent" method on the MessageDiscussion class. So your REST API call might look something like:
/restapi/vc/threads/id/45/participants/recent
You'd still have to iterate through the response to get the list of users in a format you wanted, but that could be done pretty easily with an endpoint. Here's some more information on endpoints in case you're not familiar with them:
I hope this helps!
*edit*: Looks like this was moved to the Developer forum, so I removed my comment about heading over to the Developer forum for any questions on endpoints.
