The Friends actress is throwing her 50th birthday party next month, but sources told Heat magazine the celebration could actually be for a wedding.
"She's throwing a big party at home in LA and it looks like it'll be more than a birthday bash," the source said. "She's asked guests to wear white and dress smart -- and has let slip that there's going to be a BIG surprise."
McDaid, of the band Snow Patrol, moved in with Cox in her Malibu, Calif., home in April. The two first went public with their relationship earlier this year.
"Coco's going to be a bridesmaid and David [Arquette] has been asked to make a speech. They want to put on a united front and show that everyone's happy," the source said. "Court's friend Sheryl Crow has been asked to perform at the party and Jen Aniston, Lisa Kudrow and Neve Campbell are also on the list. It's been a whirlwind romance, but they are the perfect match. They're so happy together."
Cox's ex-husband, Arquette, recently welcomed a son with his new girlfriend, Christina McLarty.
Cox and Arquette divorced in May after a three-year separation.