The comedy is currently untitled but has been known as Oh F**k It's You, Deadline reports.
The show focuses on Nick, who realises after a health scare that he is in love with his best friend Wendy.
Grunberg has now joined the cast as Wendy's brother Charlie, who is also very good friends with Nick.
Charlie, who is described as cynical, is going through a difficult divorce and counsels Nick against confessing his feelings as he remembers Nick and Wendy's previous unsuccessful attempts at dating.
Grunberg's TV credits include shows such as Heroes, Felicity, Alias and Love Bites.
JoAnna Garcia has already signed up to play Wendy in the comedy.
The project is being developed by Friends, How I Met Your Mother and Will & Grace producer Greg Malins and Dawson's Creek, Everwood, Eli Stone and Brothers & Sisters alum Greg Berlanti.
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