Group Chat, as the name suggests, enables users to chat in groups. It is a kind of chatroom which can be created by non-admin users. If a user wants to chat with 4 of his friends, not in public chatroom, but privately, he/she can create a group chat by inviting all 4 of his/her friends. Nobody else can see this chat without an invite.
For example : -
1) Mark and Stuart are initally chatting, and decide to make it Group Chat and add Sandra. Stuart is inviting Sandra to Group by clicking on invite button at the bottom of chat window. Mark's screen will look like this :-
2) And thats it Sandra will join the Group Chat and three of them can chat privately. Screen will look like this :-
Please note that this feature is available from Professional Plan Onwards.