The 35-year-old musician and 42-year-old actress attended brunch at the Four Seasons hotel with Madden's brother Joel and Nicole Richie. Joel Madden and his wife also had daughter Harlow and son Sparrow in tow.
"Cameron and Benji looked like newlyweds," a source told People magazine. "They walked around hand-in-hand. They seemed happy, too. Cameron looked pretty, but a bit tired. [She was] very casually dressed."
Richie introduced Madden and Diaz, and the couple were first linked in May 2014. News of their engagement broke in December, and the musician and actress held a small wedding at their Los Angeles home January 5.
Benji and Joel Madden formed rock band Good Charlotte in 1995, and currently perform as The Madden Brothers. The pair released their debut studio album as a duo, Greetings from California, in September.
Diaz recently appeared with Jason Segel in Jake Kasden comedy Sex Tape, and portrayed Miss Hannigan in a Will Gluck adaptation of Annie. The actress is scheduled to reprise Elizabeth Halsey in Bad Teacher 2.