Jennifer Aniston and fiance Justin Theroux have been engaged for nearly four months, but they finally celebrated their plans to marry with a crowd of A-list friends at a private engagement party in Beverly Hills on Sunday.

The couple hosted the soiree, which was described as "low key," at the home they share together. A host of celebrity pals attended the party, which seemed to have a casual vibe, as all the stars photographed outside Aniston and Theroux's home were dressed down and looked relaxed.

Some of the high-profile attendees included Ellen DeGeneres and wife Portia de Rossi, Ben Stiller, Demi Moore, Tobey Maguire and wife Jennifer Meyer, actress Lake Bell and Aniston's long time agent Kevin Huvane.

CLICK HERE to check out a series of photos of A-list guests like Ellen DeGeneres and Demi Moore showing up to Jennifer Aniston and Justin Theroux's home for their engagement party.

Aniston and Theroux have been busy getting their wedding planning into high gear, and according to a new report, Theroux is helping Aniston design her wedding gown for the big day and has attended several meetings with designers to iron out what look Aniston wants for her nuptials.

"Jen always consults Justin on her wardrobe, whether it's casual or if she's trying to pick a red carpet dress," said a source close to the couple. "She really values Justin's opinion and says he's often the only one to be brutally honest. They met with Valentino's creative directors Maria Grazia Chiuri and Pier Paolo Piccioli to discuss wedding dress designs and Justin had a big input. They showed Jen a variety of fabrics and Justin helped them recreate what he knew was in Jen's head."

Typically, it's seen as bad luck if a groom sees his bride's dress before the couple's wedding day. But Aniston's friends said she isn't superstitious and doesn't care about Theroux seeing the gown at all, even when her pals tried to convince her otherwise.

"They told her she was 'crazy' and said what bad luck it would be, but Jen's never been superstitious and said Brad [Pitt, her ex-husband] never saw her first wedding dress, but they still divorced," the insider said.