To have disciples, a Gunslinger must be of at least 8th level, true to his or her alignment, and possess the Teaching non-combat proficiency. A disciple is a lower level Gunslinger (levels 1-4), who serve under the high level Gunslinger for the purpose of gaining instruction.
A disciple is different than a mere ally in that an ally can be of any alignment and any character class and doesn't necessarily seek out the Gunslinger for wisdom or instruction. An ally is usually a friend or a comrade in arms.