Harris (The Hangover, The Jay Leno Show), 41, will play Shanna, a woman that the network describes as being the "nemesis" of Jules (Courteney Cox).
Also joining the show is Alan Ruck (Ferris Bueller's Cameron, Spin City), who will play Frank, the husband of Harris' character, who has a crush on Jules. Ruck will split time between Cougar Town and FlashForward, on which he also appears.
Both castings were first reported by EW. The new characters will first appear in November, and are likely to recur.
ABC just announced a full-season pickup for the Bill Lawrence-produced sitcom.