Start a chat with a single contact first. Once the chat has been established, click Group Chat from that tab to invite more participants.
There's no limit to the number of people you can chat with in a group chat, and any participant can invite others to join as well.
To end your chat, simply click the X in the chat tab to close that chat. The group chat will continue until all participants have left - if you want to rejoin, you'll need to be invited back by a participating contact.
****** DON'T FORGET TO CHECK SPAM FOR INVITE******