"Leneth can be recruited when you interact with the top right icon in the tasks section of the command center screen. It requires that you advanced the events with Jackie and Lexi, that you had sex with Leneth, and that at least one small and one big harpy was captured and released at the big farm. Leneth squad can fly, which allows her to ignore most, but not all, terrain limitations. She can't fly over any other squads."
From the Walkthrough p181

The first sentence is not really update but I think the rest can help you.