I first look at whether anyone responded to the "Hello everyone" that I start every dungeon off with. Responding is, effectively, required for me to give someone a Commendation. If multiple people respond, I tend to look at who "stood out" the most for me, in terms of friendliness and skill. I also factor in whether or not they're actually doing the dungeon for a non-Roulette reason; i.e., if we're doing Copperbell (Normal), and there's a legitimate level 17 player in there, I will be inclined to Commend them over others, all else being equal.