Katy Perry told TheHotHits "There wasn't really any partying, cause when I party I party! I party like Andrew W K style, I crowd surf!"
"But no we were just hanging out, I love Selena and we have a lot of mutual friends and Justin's always been so nice to me and Robert somehow got stuck in all of this!"
There's also a video at the source, but looks like it's for Australia only.
Pictures and videos of Rob from that night were posted here.