Meghan Trainor, Tori Kelly rock ‘All About That Bass’; Cody Simpson has a new friend

“Meghan Trainor was all about the jam session -- with rising star Tori Kelly -- at a private party Thursday night celebrating the latter’s debut album, “Unbreakable Smile.”

With Kelly on amplified acoustic guitar, the ladies broke down “All About That Bass” on a tiny corner stage in front of an appreciative invite-only crowd, and then the star of the evening made a genuine appeal to her audience:

“I know there’s a lot of musicians in this room,” the 22-year-old said. “I’m leaving these mics open ... I’m just saying ...”

It was a pretty safe bet: Guests included Kelly’s “I Was Made For Loving You” collaborator Ed Sheeran, who navigated through the paparazzi and fans outside the Nice Guy lounge to hit the scene around 11 p.m., straight off his second of two sold-out shows at the Hollywood Bowl. Justin Bieber arrived early and stayed through Kelly’s set, and Cody Simpson, Ashley Benson, the Weeknd and members of Fifth Harmony and Rixton mixed and mingled at the event produced by Scooter Braun.

“@ToriKelly my dreams came true tonight,” Trainor tweeted in the hours after their duet. “I always wanted to see you live and sing with you ... love you, congrats babe.”

The packed lounge in West Hollywood had quieted down around midnight before launching into supportive cheers and applause as Kelly kicked off her 40-minute set. After an introduction from manager Braun, Kelly freestyled through chunks of her own tunes and mixed in covers including “Suit and Tie” and “Say My Name.” The evening’s biggest sing-along hit was “PYT,” timed nicely in honor of the sixth anniversary of Michael Jackson’s death. Fun fact: “PYT” also happens to be Kelly’s favorite song to cover.

“I’m realizing how hard it is to invite people and continue playing,” she said, emphasizing, “Guitar players are welcome too!”

Camilla Cabello and Ally Brooks of Fifth Harmony hit the stage for a bit of “Reflection,” and Jake Roche of Rixton had his moment on the mic as well.

And, hey, was Bieber actually sitting on Braun’s lap for a minute there in their booth right next to the stage? Definitely the Biebs and Scooter in the booth. The lap-sitting detail -- up for debate, as it happened in a flash. Selena Gomez was also spotted in the room, but disappeared in a flash. Surprise?

Kelly was a gracious hostess, at one point making a happy toast to the release of her album and “to all of your being here .... I’m making eye contact with all of you right now!”

In a little black dress and flat black booties, the singer stayed until after last call and checked in with people all over the room to make sure everyone had a good time.

“All my friends are still here,” she bubbled around 2 a.m. “This is great.”

By that time, the shoulder-to-shoulder crowd had thinned significantly, to the point where the mod booths, marble-topped bar and ‘70s-’80s-mafia-fusion details of the Nice Guy could finally be appreciated.

Another famous face at the reservation-only West Hollywood venue after last call: one Cody Simpson -- who split with Gigi Hadid in May -- getting face-to-face cozy with a lovely brunet up against one of the venue’s booths

Good party.

Follow Christie D’Zurilla on Twitter @theCDZ and Google+. Follow the Ministry of Gossip on Twitter @LATcelebs.