Just to clarify, we (darkwallet) don't exactly use a centralized matchmaking service, nor did we do at any point (as I would define it anyways). We did use a centralized matchmaking server in our first proof of concept on this thread.

The current scheme works on top of any chat service, where we initially integrated a simple chat in our lobby and now it's the same lobby but the channels exist in a p2p network of all gateways so clients can connect to any gateway or gateways seed from any other gateway.

You are right otherwise we don't yet use ring or blind signatures at the moment, so restricted to 2 party coinjoins, but the general design is done so we can (more or less) easily implement more complex coinjoin protocols.

cheers!