popup messages and online users

I am developing a GUI where one member  or user in  a team  can post messages and  explains about  his problem  to the other users.

So the same application will be running on two machines. So one user logged in one system and  another in another system same  way there are  n number of users.So now the user who  wants to post the message who are currently  logged  in .So i already done that part (the users who are  already logged  in) 

so what is  my problem  when the  user log into system  i  should get some popup messages and also the user(already logged  in user) should send  some popup notification to  the user who  logged into the system about  the  message

So  can  u  please help me in  this regard