The multiplayer mode for registered users supports exchanging text messages in the lobby and in game. Below is a screenshot of how this looks like in the lobby:
One question about the guest-mode you have in mind: Does it support exchanging text messages between players?
I had a look at the link to Kongregate. I do not have the software installed here they ask for on the Kongregate page, so I was not able to see any multiplayer functionality.