"- 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."
The mentioned icon can be found when entering the command center screen by interacting with the command cente building on the country overview map. In the bottom left corner are 10 icons for "Tasks" and the icon in the top right of it, a blue icon with two people crossing spears, is for recruiting.