Sunfire is no stranger to being a member of a team other than the X-Men, as the Japanese hero has previously been the leader of Big Hero Six in addition to his time with Xavier's mutants. Sunfire was also selected and served as the Horseman of Famine for the villain Apocalypse at one time, a role he regrets and struggles with to this day.
While it appears that Sunfire is simply a fire-based mutant this is not the case - he converts solar radiation into ionized plasma that catches flame when exposed to oxygen. He can also project deadly radiation and propel himself through the air for flight.
Sunfire's invitation to the Avengers came when Logan asked him to join the Avengers Unity Squad, an Avengers branch dedicated to showing support and cooperation between regular and mutant heroes alike. Though reluctant, Sunfire agreed to join the team and would later prove instrumental in defeating the Apocalypse Twins and Kang the Conquerer by using his abilities to absorb the energy of a Celestial, making him immensely powerful.