From what I have been hearing is that it's a Republic only instance...I hope this isn't true.
"The Tion Hegemony is in utter chaos. A mysterious outbreak of the Rakghoul plague has spread across several worlds, killing countless civilians while mutating others into Rakghouls themselves. On the planet of Kaon a small group of survivors fight against the endless waves of Rakghoul monstrosities. Now both the Galactic Republic
and the Sith Empire
have set plans in motion to rescue the survivors of Kaon, locate the source of the virus, and forge an alliance with the Tion Hegemony."
Quoted from the update page. Both factions can participate.