While Deadpool has always existed outside the periphery of the X-Men universe, he has had numerous interactions with Marvel's other major superhero team, the Avengers. However, following the events of "Secret Wars," Deadpool actually joined the team for some period of time. Though, naturally, with this being Deadpool, there was a catch to Deadpool's membership.
Following "Secret Wars," Wade Wilson was brought into the Avengers Unity Division, a branch of the Avengers that initially formed in the aftermath of the "Avengers vs. X-Men" event. Captain America realized that he had, up until this point, done very little to help with the X-Men and their plight. To help remedy this, he created a branch of the Avengers designed to integrate mutants and normal superhumans alike, all for the common goal of protecting the world. However, with the proliferation of the Inhumans on Earth, Captain America eventually decided to rebuild the team to incorporate more Inhumans as well.
In Gerry Duggan and Ryan Stegman's 2015 Uncanny Avengers run. this new iteration of the Avengers Unity Division brought together Rogue as the team leader, Captain America, Human Torch, Spider-Man, new Inhuman Synapse, and returning members from prior rosters Quicksilver and Jericho Drumm. Deadpool was invited to join the team personally by Captain America.
Captain America's reasoning for bringing Wade Wilson to the team was he recognized the merits of Deadpool's character, as he once saved Rogue's life. Captain America recognized this as a testament to Deadpool's inner-hero and believed that the team would offer Deadpool a chance to be a real hero. Deadpool accepted the invitation. However, there was another, less wholesome reason he was brought onto the team. He had become incredibly popular in-universe. As such, by merchandizing Deadpool's image, the Avengers Unity Division was able to remain funded to continue its crime-fighting. During one of the first missions of this new team, Deadpool and the gang encountered Super-Adaptoid.
This A.I.M. robot was constructed of unstable molecules and a fragment of the Cosmic Cube inside it. During a fight, Deadpool watched as the Super-Adaptoid copied the Human Torch's powers. So, curious, he just touched the Super-Adaptoid. He learned that the Super-Adaptoid could copy the abilities of anything it touched when the android copied Deadpool's regenerative abilities, along with his cancer. Spider-Man was so disturbed and repulsed by this that he left the team. Though everyone -- especially Spider-Man's good friend Johnny Storm -- was upset, they didn't kick Deadpool out. They couldn't afford to lose him, since, if they kicked him out, they'd lose the ability to capitalize on his presence.
While the Avengers Unity Division did help bring these three factions of the Marvel Universe together, Captain America did have an ulterior motive for bringing the team together. Leading up to the events of "AXIS," a clone of the Red Skull had extracted Charles Xavier's brain and placed it inside his own skull. Cap wanted to figure out a way to extract the skull from Red Skull to break the Nazi's now-vast psychic abilities. However, Red Skull had possessed Quicksilver and used him to capture the rest of the Avengers Unity Division and take over their minds. Thankfully, both Rogue and Deadpool had anticipated something like this might happen, so Deadpool retrieved Magneto's helmet. With the help of both Wong and, ironically, Spider-Man, Deadpool managed to put Magneto's helmet onto Rogue's head, allowing her to break free of Red Skull's mind control.
Thanks to this, the team accomplished its mission. Deadpool chose to leave at that point, ending his tenure with the Avengers. As it turned out, he just left the team as the events that would lead up to "Secret Empire" were set in motion and, unfortunately, Hydra Cap would knowingly exploit his blind faith in Captain America to destructive ends.