Hi
This can be done, please verify why do you need this in Vb6? because it would be easy if it can be done ASP.net or PHP. No problem if you need to do this specifically in vb6.
have already made my own private chat systems.
m fluent wid vb6 as well as the new .net version.
so if ur interested let me know which platform as well as how you would like it implemented.
time could vary b/w 7-10 days depending on the complexity and features required.