The Dan'in (literaly: Gang member) are the lowest rank of Rockets. They are the Grunt men, the basic flunkies. They even took orders from Jesse and James during the heist aboard the SS Anne. When playing any of the Gameboy games (and in Pokemon stadium) these are the Rockets you run into.
In the Pokemon Adventures manga, the Rockets they run into also appear to be of this rank. The rank is classified by the simple black shirt and pants with a black hat. Rockets of higher rank have more elaborate outfits.