Before he was Ant-Man, Scott Lang already had one foot in the door to becoming a superhero. In his debut appearance from Avengers #181, Scott Lang was the engineer hired to install a new security system at Avengers Mansion.
Returning from the movies, Wonder Man and Beast were the first to test out these new defenses, as they accidentally triggered a set of mechanical arms designed to stop intruders from entering the headquarters.
But mechanical arms weren't the only security measures introduced that issue. With a ballooning number of heroes coming and going from the mansion, including the Guardians of the Galaxy, the federal government decided to limit the Avengers to seven active members.
Government liaison Peter Gyrich announced the new lineup as Iron Man (chairman), Vision, Captain America, Scarlet Witch, Beast, Wasp, and Falcon (recruited specifically to add an African-American to the group).
Of the heroes who didn't stay on, Hawkeye was the most outspoken against the government intervention. In a thought balloon, Wonder Man noted that Moondragon left without even saying good-bye.
When Falcon decided to step down in Avengers #194, Wonder Man filled the open slot (with no mention of maintaining racial quotas).
The team membership remained fairly consistent until Moondragon's abrupt return in Avengers #211.