You can add workgroup to some shared group, so every user will see it. You will have to add it with full JID (name@workgroup.server) and it will look different than other group members on Admin Console. But it will look ok in Spark roster. Though, there are some issue with workgroup's presence update, it takes long to update workgroup's presence somehow. And most of the time client needs to relogin to see that workgroup is actually online now. If you dont want to add workgroup to you roster, you can instruct users, that thye have to start chat with JID name@workgroup.server to be able to login to Fastpath queue.

As for the Agents. Fastpath is integrated into Spark. So after you add some users as agents to a workgroup's queue, they should relogin and they will get Fastpath Tab in thei Spark. Agent could be a member of several queues, but he would need to login to selected queue on Spark startup.