Diaz, 45, turned heads in a dazzling red sequinned wrap dress by Jenny Packed, along with a matching, bold lip, black bag and gold earrings.
Her 39-year-old Good Charlotte rocker husband went all black in a suit and bow-tie.
Other celebrities who on the guest list for Paltrow and Brad Falchuk's glamorous engagement party includded Reese Witherspoon, Kate Hudson, Jennifer Aniston and Liv Tyler.