Different channels may have different behaviors. Channels can be closed. Anyone can knock on the door of a closed channel. Upper limits on the numbers of users in a single channel may be configured. These can be different for different channels.
Users can be anonymous.
Normally, each line typed by a user is prefixed with their login id when it is displayed to others. Some channels can configured so that people can change their names, or to randomly assign names to users, or to give all users the same name. Or you can make it require that all users have unique names. You can set a default alias to be used in any channel that allows you to change your name. Users can be filtered out.