Could someone tell me what command do you use to chat to users who are occupied during the SSH session

You can use wall to Broadcast message or talk to chat with other logged user.

#wall 'hello' will broadcast 'hello' to everyone on the server

To make a notice to a specific user, use write username.

For example,
write root
hi hi hi

CTRL-D

Several ways:

- wall: to send a message to all logged on users
- write: to send a message to a specific user terminal
- talk: to chat interactively with a specific user
- ytalk: like talk, but it is possible to talk with more than one user simultaneously