Just a simple player ranking system like we have in the forums here would be fine. Leaders could then see group requests and the player's rating to decide if they want that player. There are lots of ways to 'rate' players, even with just the data in the my.ddo.com stuff would be a start.
Also, Turbine should do cross-server instancing. That way I can get players from all servers to run an instance with me. More to choose from = better.