How to activate group chat in Skype 4.3 in Ubuntu 14.04?
If you actually mean group chat and not video calls, write
/msnp24
in any Skype chat window, and restart Skype. And all of your group chats from any other machines and chat history will become available.
INFO: here
According to the official sources here, the cloud based group chat is not yet available on Skype for Linux. However you can still do a conference call by clicking on Options and selecting Start Conference Call.