AFAIK, there is no implementation beyond some proof-of-concepts, but the idea is that Matrix (via bridges) can receive and send activitypub updates and treat them as messages in a room. So you could follow and respond to people on a room and they would receive updates on their ActivityPub server. It

This idea is not new, really. Movim (https://movim.eu) is running a federated social network on top of XMPP for over a decade already.