The X-Men are a group of freedom fighting mutants. Deadpool tried to join them a few times but Cyclops refused his offer and even sent out Domino to kill Wade. Although in the end, Cyclops saw Deadpool's potential as a part of the cause, and assigned him to Wolverine's team, X-Force.
Wade did his best to impress the X-Men, even by killing Mercury's father who was using his mutant daughter whom he barely spoke to to gain some public attention to sabotage the X-Men. After this incident the X-Men reluctantly accept Wade Wilson as an honorary X-man.
Deadpool met the X-Men in the following comics:
Deadpool (volume 3) #16-17 X-Force Annual #1 (2010)