The 28-year-old 'Firework' singer - who split from the 35-year-old singer/songwriter for the second time in March - invited him to celebrate Memorial Day with her and several other friends at her new Hollywood Hills home, where they appeared to be back together.
A source told UsMagazine.com: "They seem to be back together -- or back together-ish. They were very flirty and having a really good time together. They have good chemistry."
Another eyewitness added: "They were definitely friendly."
Other guests at the party included 'Girls' star Allison Williams and her boyfriend Ricky Van Veen, who were spotted on double dates with Katy and John in the past.
Comedian Mindy Kaling, BJ Novak, '2 Broke Girls' star Beth Behrs and 'Raising Hope' actress Shannon Woodward also showed up to toast Katy's new home.
A source previously predicted that the on/off couple might reunite following their split in mid-March.
The insider said: "It's not over until it's over. You have to see how things play out."
Katy spent the rest of the holiday weekend with Robert Pattinson in Santa Barbara.
The 'Twilight Saga' hunk - who recently split from Kristen Stewart - and the 'Part of Me' singer - puzzled onlookers when they were spotted checking out the wedding rehearsal of two strangers at San Ysidro Ranch, not far from where Katy grew up.
Rehearsal guest Stephanie Sands told People.com: "Katy and Rob were just sitting next to each other watching the wedding rehearsal in the main courtyard area. They were dressed very casually. She was wearing a hoodie and big sunglasses. There was no PDA [public display of affection] between them and no body language that showed they were dating. They just sat together and watched.
"They were not affiliated with the wedding party and were not seen mingling with guests."