Unfortunately the webchat plugin is a bit out of date and has some minor issues like that ( I'm working on updating it slowly ).
To access a workgroup you can go to http://host:port/path/start.jsp?workgroup=workgroup_name@service_name.server_nam e.tld obviously replacing the placeholders. This will get you to the workgroup form to initiate a chat.
you can find the value that goes in the workgroup= part on the workgroup settings page in openfire, would be something like myworkgroup@workgroup.example.com