Simple concept, brilliant game
This game feels like you just sat down one day and thought, 'hey, what would happen if I put a whole bunch of people in a room and let them edit it however they wanted?' And it works perfectly! The pure simplicity of it is what makes it fun, and I think that giving it a chat function would only reduce that fun.
I do have two suggestions, though:
One, make some kind of indicator to show which smiley is doing the editing. For example, make a line drawn between the smiley and the block, or, if that would make the screen too cluttered, make the smiley flash the color of the block they're creating when they create it.
Two, to deter trolls and troublemakers, implement an attack system where you can damage someone by clicking them and kick them out of the room or give them some other penalty (like make them unable to edit for a short period of time), but make it impossible for a single person to knock someone out (for example, make them regenerate faster than damage can be done, so multiple attackers would have to agree on a target for it to work). Since the troublemakers are often single people who can do a lot of damage against the wishes of many, this might help to limit the damage they can do.
Even without the attack system, the creation indicator should still help, since at least it would show you who's doing the damage and let everyone team up to push them out of the way with arrow blocks.