To activate (deactivate) chat option for the group go to workgroup Actions > Edit Workgroup:
Then click More and tick Chat in Features section:
The chat option will be listed in the workgroup features top panel. You can also easily access this chat with the help of special chat icon located in the right area:
The workgroup chat is a private chat consisting of workgroup members (all group members are added to the chat automatically):
You cannot add new users to the workgroup chat, you need to add them to the workgroup instead.