This a list of guys you can invite into the game.

Unlockable	How to unlock
Aegis	Complete all of Tomix's quests, and then select "Become a SoulWeaver". Choose the quest titled "Meet Aegis". Finish the quest, then become a SoulWeaver (bring Tomix 3 Shards of the SpiritLoom and you can become one). After you become a SoulWeaver, you can invite Aegis.
Artix	Go to Amityvale or Doomwood (5-30dmg/Holy 10-200dmg)
Ash	In the center of Falconreach (5-10dmg)
Captain Davey	Go to the Locker and do the Aquella's "Aboard alone" quest.
Cinder	(After you get Goldfish) Talk to Grams in the Pet Shop and say Pet Quests, second quest
Cranix	Press travel and go to battleon. Press fire war. Press battleon burns. Press bring along guests. Press boss battle and during all the talking teleport to home town. NOTE: He's only in slot 2 (lvl 16-mage-dark-doom wep)
Galanoth	In Dragsvard (15-25dmg)
Goldfish	(After you rescue Aria) Talk to Grams in the Pet Shop and say Pet Quests, first quest
Konnan (The apprentice)	go to the griffin bird near the gaurdian tower then click on a place called "Anomaly"
Mennace and Elgert	Go to BattleOn and do the quest just before BattleOn Burns. Then you will get a Doom Knight (Mennace) for slot 1(76-82 damage)and a DragonLord (Elgert) for slot 2(63-75 damage)
Nythera	Go to Warlic's zone and do Nythera's quest "The storm war" then "Diplomatic Mission" and finally play as her in "The Final Showdown".
Robina Hood	In Surewood Forest in the far right of Falconreach (5-10dmg)
Sir Ano	Go to oak lore and do the level 10 quest and when your in the quest go to ,your home town and boom you have sir ano.
Snailmail	(After you get Cinder) Talk to Grams in the Pet Shop and say Pet Quests, third and final free quest
Tomix	Go to Ravenloss and choose the alley quest. After you talk to him, he goes into friend A slot. Don't do the quest just go back when the do quest/back buttons appear.
Twilly	Falconreach, sitting on a tree stump
Valencia	In the far right of Falconreach past the sign that says That Way. (8-18dmg)
Warlic	Go to Warlic's Zone and click on Save Lymcast and do Xan's Fortress and then go back (15-20dmg/ice 20-30 dmg)
Xor Vralin II	Press travel and go to battleon. Press fire war. Press battleon burns. Press bring along guests. Press boss battle and during all the talking teleport to home town. NOTE: He's only in slot 1 (lvl 36-warrior-light-destiny wep)
Young Artix	Go to the Necropolis and do the quest "the green mist"
Young Nythera	Go to Warlic's zone and Nythera's "It's Not Fair!" quest.
Zhoom	Go to the Sandsea and to Zhoom and click Quests and Sek Duat Lives again, then run away (15-25dmg/2fire 20-40dmg)
Zorbak	Suewwould forest- After you see Robina, go to the right and down. Must be level 6 to invite.