MUSHes have a variety of communication formats, from paging (direct messaging player to player) to channels (globally accessible group chats) to in-room roleplay posing.
The first thing you'll probably want is the public channel. Once you've connected to the game, as a guest or a character, type: =pub Hello!
That will get the attention of whomever happens to be hanging out on the game at the time, and you can carry on a conversation by prefixing your replies with =pub to send them to the global public channel. The =new (Newbie) channel is also available.
You can also 'page' individuals: page Bob=Hi! This sends them a private message to which they can reply in kind.
Finally, when in the same MUSH 'room' (a simulated area), the prefixes : and ; are used for 'posing', e.g. :nods. would result in Bob nods. being shown to everyone currently in the same room. (The ; prefix is used when there should be no space, e.g. Bobnods.) Posing prefixes can be used within pages and channel conversations as well.