Spoilers for the end of act 3 below.
Idk what happened. I’ve only ever helped her. We just came from the astral plane and are ready to fight the brain. She pledged her sword and I gave a speech saying everyone is the hero of Baldur’s Gate or some such, and then it cut to her telling me off and telling me if I die against the absolute it’s better than I deserve and she left. WTF? Does anyone know why that happened?
Edit: mystery solved. More spoilers below.
If you kill Minsc instead of saving him, and then don’t talk to Jahiera before the battle, she finds out about it after your speech, is hella pissed, tells you off, and leaves. She already pledged her harpers though, so you still get to use them.
Yes, you have to do some quests first. I think it was around mid or late Act 2 when he got usable as a member. I am not sure which quest, likely the questline regarding the sick man in the Last Light Inn / little boy and his ghost.
I didn’t even know that you could do a quest for him the first time 'round, because the trigger to start it is so easy to miss. I only found out because a friend told me.