Richie and her husband Joel Madden were on hand to see author and style blogger Katherine Power marry photographer Justin Coit at an L.A. registry office before a reception at renowned restaurant Mr Chow.
A heavily pregnant Alba also attended with her husband, Cash Warren.
After the ceremony, Richie toasted the newly-weds in a post on her Twitter.com page, writing, "Congratulations to the new married couple Mr. & Mrs. Coit!"
Other famous guests included stylist Rachel Zoe and DJ Samantha Ronson.