In Grand Theft Auto: San Andreas, the feature of recruiting gang members was added. This feature was added again in Grand Theft Auto IV, but can only be unlocked if Dwayne Forge's freindship is high enough.
Grand Theft Auto: San Andreas
To recruit Grove Street members in Grand Theft Auto: San Andreas, you must aim a gun at them and then press a specific button in each version to recruit a member. They will kill rival gang members and kill anyone you attack. They will also attack cops if they are chasing you. Gang members can also take photos of you. Its possible to activate a cheat to recruit anyone (including rival gangs) to join you. For example, if you recruit some Ballas, they will attack Grove Street and Varrios Los Aztecas members. A rare glitch affects this cheat were if you recruit these gangs they will attack their own gang.
Grand Theft Auto IV
The only way to get backup is to get Dwayne's relationship high enough. Once that is achieved, he will send backup on request. 2 gang members come, and will assist you. The gang members will only attack other gangs if they attack you, and will also attack cops, but only if they attack you.