Choose a complex secret password, then share it with the persons who would you like to chat with - preferably via phone, but for paranoid security level you should meet them face-to-face and remove batteries from all cell phones during the conversation.

Good to know before entering a chat room

Password and room have to be the same for the participants, username can be anything. Room and username doesn't have to be handled as a secret information (unlike the password). Joining a room with an incorrect password will end up with a bunch of random characters instead of plain text messages.