The two couples were among the crowd of revelers at Britney Spears's New Year's Eve concert in Las Vegas on Wednesday

By Liz McNeil
Updated January 01, 2015 02:30 PM
Credit: Gabe Ginsberg/WireImage

For their first post-engagement outing, Sofia Vergara and her fiancé, Joe Manganiello, rang in the New Year along with a group of celebrity friends at the Planet Hollywood Las Vegas Resort and Casino.

The couple, who got engaged in Hawaii over Christmas, were joined by Miley Cyrus and her boyfriend, Patrick Schwarzenegger, as well as Patrick’s dad, Arnold Schwarzenegger (who co-starred with Manganiello in the film Sabotage), and his girlfriend, Heather Milligan.

“Miley and Patrick walked hand-in-hand before they caught up with Arnold at the Strip House,” says a source, adding that the young couple were “all smiles” and that they later stopped to take selfies with fans. The group then had a private meeting with Britney Spears before her Pieces of Me show.

Vergara and her fiancé, meanwhile, “were very affectionate with each other and were just beaming,” a source tells PEOPLE. “You could tell they were newly engaged and so happy.”

The pair were eventually joined by Arnold and his girlfriend and other friends, and sat front-row at the concert. The Modern Family star was spotted taking pictures, and posted happy snaps of her celebration with Manganiello. (Iggy Azalaea was also in attendance.)

Their famous friends also had a good time as they sat beside the stage. “Miley sang along to every word and chatted with Britney between songs,” says an eyewitness. Patrick snapped photos and took video of his girlfriend, as well as the concert. Says the source, “She and Patrick were on their feet the entire time.”

And after the clock struck 12, Cyrus posted a picture of her New Year’s kiss with Patrick.

With reporting by MARK GRAY