U can get them under special circumsdtances eg. Edwin finding the nether scroll happens as soon as u enter the graveyard district with Edwin in your party, Keldorns Family quest is given to you when you enter the government district with keldorn in you party, (SPOILER) Free Yoshimo starts practically as soon as you are caught by Irenicus In spellhold.
others You get as soon as the NPC joins your party eg. Nalias keep Quest, Korgan Finding the Book, Valygar- Planar Sphere
Others You Need to Do a quest just to get them in your party in the first place- HaerDalis- Rescue HaerDalis, Viconia- Rescue Viconia from the fanatics, Keldorn- Get the Quest of the Unseeing Eye, Edwin- Complete his and MaeVars tasks in the thief stronghold quest, Aerie- Rescue the Circus, Jan- save him from the guard that arrests him for peddling.
Others You have to have them in your party AND THEY OCCUR AT RANDOM TIMES- Mazzy- Family Quest,Jaheira- HArper Quests, Nalia- Fathers Funeral- Kidnapping, Anomen- NIghthood Quests, Aerie- Summoned to the circus
This should cover all the quests asotiated with party joining NPCs...
so bascially in answer to the original question is It happens at random times.
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