Hi,
iChat was the forerunner to the Messages app.
Since iChat 4 it has been able to do 4 way Video chat (you and three others) and 10 way Audio Only chats.
It could also do Screen Sharing from within the app before it was added as an invocation like Face Time in Messages.
However currently AIM and Jabber accounts (the ones you should be able to use in Messages) are unable to do this and Apple have an issue with the server that does the connecting.
This is effecting all iChat users as well as Messages users in Mavericks, Yosemite and El Capitan.
Strangely LAN based chats still work.
This is normally the Bonjour account but can be done with AIM (If you have two Screen Names to use on two computers).
It looks like this:-
Other than that the question begs for speculation which the Terms of Use forbids.
10:05 pm Thursday; October 29, 2015
iMac 2.5Ghz i5 2011 (Mavericks 10.9)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad