Cameron Diaz and Nicole Richie: Banger Sisters At Good Charlotte Concert

There's nothing Cameron Diaz and Nicole Richie don't do together these days -- including partying like teenagers at their husbands' concert.

The sisters-in-law fangirl'd out in the VIP section of Good Charlotte's Thursday night gig at the legendary Troubadour. While Benji and Joel Madden rocked onstage ... the ladies were rocking beside Nicole's dad Lionel Richie.

Just a couple days ago, Cameron and Nicole shopped for shrubbery together ... so, they're definitely in the BFF zone. Check out the clip, Nicole seems less into the performance than Cameron, but she's been at this longer.