The film, starring Natalie Portman and Ashton Kutcher, is from the former Blacklist script "F***buddies," by Liz Meriwether, about two friends who find themselves falling for each other after years of no-strings-attached sex.
Thirlby would play Portman's sister, and Greta Gerwig (breakout star from "Greenberg") is believed to be playing the role of Portman's friend who suspects Kutcehr actually has feelings for her. Kevin Kline is on board as well, presumably as Portman's father.
The film changed its name from "F***buddies" to "Friends With Beneifts," for obvious reasons. It's now untitled due to the glut if "FWB" projects floating around, including a Justin Timberlake romcom and forthcoming sitcom on NBC from Michael Weber and Scott Neustadter, writers of "(500) Days of Summer."