To "master" a crew's routines involves getting five stars on all of that crew's songs. The crews have eight or nine songs, and you'll have to get five stars on every song on Hard to unlock all of these achievements. Difficulty achievements do stack, so playing through on Hard will net you all of these achievements. Getting 16 stars for a crew will unlock their final challenge (which you must also get five stars on), you must get at least 32 stars and complete their final challenge to unlock their outfits, and then get every star to master their routines.
Getting five stars on every song on Hard will be the most difficult part of this game, so just keep practicing and utilize Break It Down as much as possible to learn all of the moves for each song. Use Video Record to try to nail down what you're doing wrong.
Also, there are a couple of really good Youtubers that post videos of Dance Central/DC2 songs on hard, which might make it easier for you to see how the dance is supposed to look. Riffraff67 has already started posting some songs on Hard on his channel. It is also highly recommended that, once you get the "Latest Model" achievement for having 1000 pictures taken, you turn off the freestyle section in Perform It mode. The Crew Challenges don't have a freestyle section; the song is just one long string of dance moves, so removing the freestyle section while you practice in Perform It mode will allow you to be better prepared.