How does one recruit her? I have checked the wiki and reddit but can't find anything. I triggered it once long ago and have no idea how to do so again.

You have to progress quite far in the story (after the underground hall?), then take the new 'dialogue' choices with her twice in the slavers guild. Finally pay off her 7000 debt in the mages guild. I'd have added it to the wiki, but not sure exactly what triggers it :(

I am at the end of the quest line and still nothing. Part of me wonder if you need to do a certain amount of business with them or something?

I believe you can actually miss your chance to purchase Maple. As far as I can tell, if you don't ask Maple about fairies during the early quest to bring Melissa a fairy, you'll never get the opportunity to purchase her (currently).

You can actually buy her quite early, I've seen her purchase available during the Taurus milk quest. I start over a lot...