Spoilers perhaps.
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I asked it as well, quite some time ago. Generally, the party member, as it seems (and from personal experience) has to be in the party for 1 in-game week. Some need to be for 2 weeks.
Some of them have quests about what they will tell immediately (like Khorgan), some of them you can get while doing the quest and they don't seem to have any other, or at least anything that big attached to them (like Keldorn - Unseeing Eye big and main quest, family issues smaller and personal and nothing after that).
In any case, you should wait for week at least to get most 'personal' quests to start with the most NPC's, if nothing happens during a week, then wait 1 more week..
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