You will have to pass a series of tests by doing exactly as they do. The test consists of three missions. Each character will show you first what to do. If you pass the test the Rudie will join your gang. Each character has a different level of Technique, Power and Graffiti that is displayed on the characters select screen.
Some characters are better at things than others.
Technique will tell you how good a character is at tricks while Power will tell you how strong a character is and graffiti will tell how good a characters tagging skills are. It is most important in the game to have characters with a strong graffiti and technique as these skills amass the most points. Test each character out around a level before you use him or her to see if you like the way they operate. Beat Beat is the leader of your gang the GG's. You start out as Beat in the game.