When They First Met
Audrey was looking for a third group member when she found Xander and you can say it's "band mate at first sight". She asked him to join the group and he pleasantly agreed. This is when the friendship began.
After Audrey and Dean saw no one that could be in the band, Xander auditioned and impressed them highly with his drumming skills. He instantly became a part of Paramore because of that.
The stolen kiss
At one point after Audrey met Melissa, Xander walked Audrey to her car, you know, just the nice, formal thing to do, just then, Xander kisses Audrey, Audrey liked the kiss, just she knew they couldn't be together because Dean is with Audrey.
The confession (sorta)
Audrey confessed her love to Xander, singing Teenage Dream by Katy Perry, Xander didn't get why she sang it yet, nor does he know Audrey likes him as much as he likes Audrey.