From Bethesda Game Studios Forums
#1. Butch, Complete the Quest: Trouble on the homefront and be sure Butch is alive, and the Overseer has stepped down or died, or you've sabotaged the Vault. Goto the Muddy Rudder bar in Rivet City, and easily convince your old "friend" to join you.
#2. Sergeant RL-3, Purchase him from traceling robot salesman called Tinker Joe who randomly roams the Wasteland near the RobCo factory. He will only follow you if you're Neutral.
#3. Fawkes, Although not able to join you immediately, once you're back in the Wasteland after visiting th Vault 87, Fawkes is waiting for you, helps kill the enemies that want you dead, and then offers to swing his gaint hammer as often as you'll let him. (hes a super mutant)
#4. Star paladin cross, Head into the Citadel for the first time and speak with her. If you're pretty up there in karma, she agrees to accompany you on a special detachment mission. If you become neutral or lower, she'll ditch ya and return to base.
#5. Jericho, Hes moping around Megaton being a pain in the ass, and only if you're evil can you talk him into resuming his previous career as a merc. He needs "supplies" though.
#6. Clover, Gain entrace to Paradise Falls by nonviolent means, speak to Eulogy Jones, and he offers to sell his companion to you. her near-brainwashed condition enables her to believe you're Eulogy's replacement, and her love is trasferred to you. (Bow chika Bow WoW)
#7. Charon, This large Ghoul has nothing to say, until his contract with Ahzrukal in Underworld is either sold or bought, This either costs you in Caps or Karma. Once the contract is in your possession, you might want to stand back, the two Ghouls dont get along.
#8. Dogmeat, Located directly south of Minefield and north of Super-Duper Market in a place called Scrapyard, you'll know hes near by the sound of the raiders shootin at him.