If your school or mainframe does not allow it, do not try to sneak its use, because it is very easy to detect. One important thing to mention is that not all nodes allow interactive chat. Some nodes are simply not advanced enough for it and you will a receive a message telling you this whenever you try to chat with them. However, this situation is less common. The command to send messages depends on your computer and system software.
Many people find receiving messages in all upper case to be extremely annoying.
For more information on the TELL and SEND commands, you should consult your local system documentation. Unfortunately there is a downside to everything and Bitnet Messages are no exception. Text sent by message must be short. In general, your message length can be one line, about the width of your screen. In other words, you won't be sending someone a copy of Phrack World News via the TELL command.