Open chat using enter, or whatever keybind you set opening chat to.
Type "/" and then the command name.
For some commands you can then type in the name of one of the people in-game.
Close chat by pressing the same button you used to open it.

The command you typed should run.

• debug - Toggles debug information on the right side of the screen. It shows other information as well, such as if any power-ups have been used (true/false) and if the rock was the only tool used so far.
  
• seed - Shows the seed of the current island in chat and copies it to clipboard.
  
• ping - Pings the server, and sends a message in chat "Pong".
  
• kill - Instantly kills you.
  
• kick [username] - Kicks the specified person from the lobby. If left blank, attempts to kick you
  (you can't kick yourself, instead an error message is shown).

I got the first 4 commands from:
https://steamlists.com/muck-all-chat-commands-for-muck/
so credit goes to them.

If you know of any other chat commands in Muck, leave a comment and tell me.