Rather than build the infrastructure for multi user chatting, you can utilize the Google Talk Gadget or Windows Live Messenger API
The Windows Live Messenger Library is a client-side Javascript library that enables third-party sites to seamlessly add instant messaging capabilities with Windows Live Messenger - http://dev.live.com/Messenger/
The Google Talk Gadget lets you see your contacts and chat with friends right on your homepage, and it doesn't require a download. You can even add the Google Talk Gadget to your own webpage or blog by pasting a single line of code into your site -
http://www.gmodules.com/ig/creator?synd=open&url=http%3A//www.google.com/ig/modules/googletalk.xml&pt=&context=b&synd=open&lang=en&.lang=en&country=us&.country=us&cat=all&num=24&start=0&cols=4&objs=w,mO,jyq,gQq,jhP,NL,Hg,pV,RB,p,33G,EKT,6aZ,7Wu,aag,2C,vB,sMg,j0,xQO,5WIK,Rm,gP1,acyU&sn=2C&lang=en