The threesome recently dined together at Jen's fave LA hotspot, Sunset Tower Hotel.
Everyone knows that if you're going to marry a gal you need to get the bestie on your side, and Justin appears to be right up there in Courteney's good books.
"Jennifer looked adorable," an onlooker told People. "She looked so pulled-together and very chic. She was dressed casually but in heels."
It's not the first time that the three of them have dined together. Jennifer, Justin and Courteney also went for a night out at Soho House in West Hollywood last month.
Jen is currently taking a break from filming We're The Millers with Jason Sudeikis and Emma Roberts in North Carolina.
"They were very calm and carrying on a regular dinner conversation," the insider added of the recent group date. "Jennifer had a big smile on her face. She and Justin both looked very happy."
Also at the Sunset Tower Hotel on Monday night was designer Tom Ford, who was celebrating his 51st birthday. He was sitting near Tom Hanks and Rita Wilson, who went on a double date with Jen and Justin back in April.