Leaf - Joins automatically during the story.
Dorothy - Select the first option when talking to her ("弟子にしてくれ" or something; has a fairly low MATK(?) requirement), then talk again and select the option that replaces it. Kill the special enemies one screen north of her house, one screen south of her house, and the first screen of the ranch. Talk to her again and report.
Victoria - enter the backdoor of the boss room of the city filled with girl zombies. Talk to her several times, choose the first option (soul transfer) when prompted. (costs 5000 souls).
Jeanne - Obtain her ashes where you fought the intro boss. Obtain an item dropped by the boss in the poison marsh northeast of the forest. Find an NPC skeleton shopkeeper in the skeleton labyrinth (northeast of the destroyed town). Talk to him twice, and with the materials in hand talk to him again("Revive Jeanne" or something). Then talk to her and select the first option (soul transfer).
Catherine - After progressing someway through the game, talk to her and her escorts in the hub (unsure if this step is needed). Progress slightly again; when they are gone from the hub, head to the skeleton labyrinth and find her. Kill the two enemies, then talk to her; select the first conversation option ("It's dangerous here" or similar). Head to the destroyed town's cathedral (boss room). She should be there. Talk to her and select Summon Jeanne, or something like that (Jeanne must be obtained as a companion).
Elma - The Pied Piper of Hamelin in the sewers must be killed. Meet her in the town with female zombies first (unsure if this step is needed). After reaching the gate of Lost Empire (the gate with 4 locked crystals), go back to her house. Reach and kill the enemy before it reaches her; if the Pied Piper is dead, she should be in the eastern part of the hub. Talk to her until she joins.
Goose - Rescue her from the ranch with one key (this step MUST be done early in the game; right after finishing the junk forest may be a good time). Deposit a decent amount (~10k or more?) of souls in her bank service. She should be gone after killing/contracting one of the four Princesses; meet her in the tent in the marsh northeast of the forest. Select the bottom option ("許さない") three times, then select the top option ("傍にいる").
Elizabeth - After progressing someway through the game, she should be loitering in the northern part of the hub. Talk to her, and then head to the junk forest; an enemy in the same area as the talking flower will drop a ring. Talk to her, and give her the ring (first option?). Talk to her again in the hub center, and choose the third option. Don't tell Red Hood to talk to her mother.
Miranda - Get 5+ Karma and head to the underground poison lake (frog princess area). If the atmosphere and BGM changes you're in the right place. Find and defeat her (note: attacks cannot be dodged or parried, frequently counters physical attacks). Choose the first option.