I hold weekly Rikti mothership raids every Thursday evening (with the exception of holidays) on the Excelsior server, at 8:30 P.M. ET/5:30 P.M. PT. We will meet in Pocket D, and begin forming the league fifteen minutes in advance. You must be level 35+ and have the Member of Vanguard badge to participate.
When the Rikti mothership raid begins, we will start with Pylon 11, going in a roughly clockwise formation. When Pylon 1 is destroyed and the Rikti mothership's shield drops, we will stay at the Pylon 1 location and buff up. Once buffed up, one or two people from every team will plant a bomb on the mothership, and everyone else will head directly to the bowl. Once the Rikti mothership raid is over, we'll begin the Rikti dropship hunt.
For more information on the instanced Rikti mothership raid trial, see here.