Katy Perry and John Mayer, with Allison Williams (inset)
Larry Busacca/WireImage; Inset: Jon Kopaloff/FilmMagic
02/11/2013 AT 05:00 PM EST
and her Grammy Awards date
star Allison Williams, made it a couples' affair at the Warner Music Group after-party.
Perry reunited with beau John Mayer
while Williams reconnected with boyfriend Ricky Van Veen as the group later huddled with Selena Gomez
, who arrived around 1 a.m., and Lana Del Rey for early morning drinks at West Hollywood's Chateau Marmont Hotel.
The party, where guests were offered Prosecco and Heineken beers, along with shrimp, quiche and sliders, was sponsored by MINI and put on by InStyle
magazine (which, like PEOPLE, is part of Time Inc.).
Perry, who called "Kelly Clarkson winning" her fave Grammy moment, praised her Grammy escort Williams. "She's such a good friend of John and [mine], and people were saying she and John could be dating, so she and I decided, 'We should go on a date together!' And now she and her boyfriend are out with us and we're having a great time."
Mayer, however, remains her main man, at night and round the clock, Perry says. "He's a better date no matter what time of the day!"
Perry turned up in Gucci for the party, retiring a mint green '70s-inspired, cleavage-baring gown
that turned heads along the Grammy red carpet earlier in the evening. She sipped Jameson Irish whiskey and ginger ale ordered by a friend and joined in on a smoke as she and pals wound down the big evening.
Perry even offered Williams a change of wardrobe so she, too, could be comfy. "She makes me smile with just a wink, and she lent me a dress when I wanted to change mine and a different one for the after-party!" Williams told PEOPLE.
As Perry's boyfriend, Mayer
, bow-tie undone, held court drawing laughs from the group, the pop star also spent couch time chatting with Gomez, laughing and holding hands until the venue's lights were turned on – signaling the event was over after a 2 a.m. last call.
They left for a private party around 2:30 a.m.