Speak to Ysiel Windsinger at the Cenarion Refuge in Zangarmarsh.
The Cenarion Circle's main expeditionary force in Outland is known as the Cenarion Expedition. Though it began as a small venture controlled directly by the Cenarion Circle, it's become a large and fairly autonomous body.
The expedition's leader, Ysiel Windsinger, has asked us to direct volunteers to her outpost. Report to her at the Cenarion Refuge in eastern Zangarmarsh. Take the main path to the west and it will lead you directly there.