If you dont like netmeeting then look at my site there is a small program which can help you in this regards. You can use that to communicate with any H.323 enabled application including. you can make multiple instance of it to make the desired effect.
As for cross platform is concered then you can try some software in Java made using JMF. if you want some more information then just email me. my website is http://www.geocities.com/funmaya/
in the software section you can find that application. Its not great application. its in its most basic form. but the source code is included with it.
But still your best bet will be some software using JMF. on java site you will get some basic programs on JMF which can be extended to your need. but either you will have to do it or hire someone who can do that