This may or may not work depending on the security preferences of the machine, but usually, you can use these commands even if you do not have sudo.

To broadcast a message to all users:

wall hello

All users currently on the machine would receive:

Broadcast message from root@domain (pts/2) (Wed Apr 6 12:53:22 2016):

hello

To send messages directly to a user:

who
winstonc pts/247 Apr 6 12:04 (107-46-41-155-wireless1x.bu.edu)
huyle333 pts/249 Apr 6 12:07 (162-38-41-155-wireless1x.bu.edu)

Let’s say that I’m winstonc, and I want to send a message to huyle333. I take his username and pts/#.

write huyle333 pts/249

I write the following and hit enter each time.

hi
WHAT
THE

huyle333 recieves:

Message from winstonc@scc1 on pts/122 at 12:55 ...scc1:/project" 12:55 06-Apr-16
hi
WHAT
THE