The actress -- who announced she was pregnant on April 28 -- showed off her growing baby bump in a Preen dress and strappy black stilettos; Jones was joined by costars Rose Byrne, Kevin Bacon, James McAvoy, Zoe Kravitz and Michael Fassbender.
The Mad Men star, 33, has yet to reveal the identity of her baby's father, though a source recently told Us Weekly that it isn't her ex, Jason Sudeikis.
"She told him she wanted kids and it turned him off," the source said of Saturday Night Live star, 35. "Jason was bummed out she was on the mommy track. If Jason were the dad, he'd step it up."
Tell Us: Are you a fan of January's maternity style?