What can I use for multi-person, single conversation, instant messaging in a serverless local network?

We have a number of Mac's in a small, simple local network at the office (all running Snow Leopard or later) that I'd like to use for instant messaging with multiple participants.

The trouble is, at present, they are not internet connected and we have no server machines, so I'm using iChat with Bonjour networking to chat with other users. This all works fine, apart from the big problem that you can only chat with a single user at a time in the same conversation (you can talk to multiple people in separate conversations on a one-to-one basis, but this isn't ideal).

Is there any way of configuring iChat that I haven't considered? Or another instant messaging client that will allow me to chat with multiple participants, in the same conversation, in a local network without a central server?

EDIT: Further reading suggests running the iChat service on a OS X Server machine within the network. Is this the only way? I was hoping not to have to add another machine just for this.

You don't need the iChat service. Any Jabber (aka XMPP) server will do (the iChat server in OS X Server is also one, basically), and it can run on virtually any machine in your network.
I'm afraid I can't recommend one, but Google should be able to help you.