Each workgroup has a set of features that can be either activated or deactivated for that group. Workgroup chat is one of such features.
To activate (deactivate) chat option for the group go to workgroup Actions > Edit Group:
Then open Features section & go to Chat parameter (green tab means the feature is activated for the group):
The chat option will be listed in the workgroup features top panel. You can also easily access this chat with the hep of special chat icon located in the right area:
The workgroup is 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.