I would have to say my fraternity, and all of Greek life in general, is its own discourse community.  Swakes gives his criteria for a discourse community, and i believe Greek life fits it.
his first criterion is that it have a set of goals.  Greek life has this in spades.  We all have an agreed upon GPA we have to meet, we have community service requirements, and we all strive to better ourselves.
We also have a means of communication.  We have a messaging service we us to talk to each other.
We also have a method of feedback when we exchange dialog with each other.  Another thing that makes us a discourse community is the fact we have membership fees, as we all use many methods of communication to communicate.  Also we have specialized words not found outside of Greek life.  The last way we are discourse community is the at we keep a limited amount of members.