The first thing that come in to mind is
on page, check for current user has new message by ajax with a interval one second. And pass his/or her message to server with a send message function. But this makes really so many request as the online user numbers grow up. And it also will harm explorer at client side as if he/or she talk to many users.
Is there any other possible methods to send message from server to client without a request taken. Or simple how goggle make chat from gmail page. I like smthg like that. I do not want to use or copy a know code to my project. So do not refer any open source project, only tell me about methods that could be available and give explanatory code examples.