The Founding Members
First Appearance: The Avengers (1st series) #1
Members: Iron Man, Thor, Ant-Man, Wasp, Hulk, Captain America
As stated before, the Avengers followed the example of DC’s Justice League, assembling characters who were starring in other titles – Thor in Journey into Mystery, Iron Man in Tales of Suspense, Ant-Man and Wasp from Tales to Astonish and finally the Hulk from his eponymous title 
the Incredible Hulk. The impetus for these characters uniting was Thor’s villainous brother and main antagonist, Loki, who manipulated the Hulk into looking like a villain, as to draw Thor into conflict with him. In the end, however, the assembled heroes foiled Loki’s plot and decided to continue working together for the common good. It was the Wasp who suggested what they should call themselves: the Avengers.
Assembling a team is one thing; keeping it together is another. Right away, the distrust of the Hulk demonstrated by his new teammates led to a misunderstanding and the Jade Giant leaving the team in only the second issue. Rather than write him off, the Avengers felt that the Hulk was their responsibility and, for the next several issues of the title, the team attempted to locate their former teammate, which ironically led them to a new one. Another change in #2 was Ant-Man’s transition to his new Giant-Man identity, which he had recently adopted in his and Wasp’s Tales to Astonish title. Not only did his new size-increasing powers help differentiate him from those of the size-shrinking Wasp’s but his new strength helped replace that of the departing Hulk. All of these changes in the second issue of the series showed that the only constant in this new Avengers series was change.
During the team’s pursuit of the Hulk, they ran across the Sub-Mariner and, in doing so, inadvertently set a chain of events leading to the thawing of Captain America, who had been sealed in Arctic ice since the end of the second World War. Rescued by the Avengers, Cap was welcomed by the team into their ranks as they helped him adjust to this new age. Given how early he appeared in the series (Avengers #4), many fans over the years came to consider Captain America an “honorary” founder of the team, a consideration eventually incorporated into the narrative of the series itself decades later. For his part, the Hulk retained his Founding Member status as well, though he would not officially rejoin the Avengers again until Jonathan Hickman’s roster in 2012 (coinciding with his inclusion in the movie that year).
The next apparent hero to join the team was a new character called Wonder Man in #9. However, this was in fact a plot by the Avengers’ current major foe, Baron Zemo and his Masters of Evil. In the end, Wonder Man repented his part in the scheme and sacrificed his life to save the Avengers. However, comics being comics, Wonder Man eventually found his way back to the land of the living years later and was officially inducted as an Avenger in #194.
Cap’s Kooky Quartet
First Appearance: The Avengers (1st series) #16
Members: Captain America, Hawkeye, Scarlet Witch, Quicksilver, Avenger X (retcon), Swordsman, Goliath, Wasp, Hercules
Despite the all-star nature of the original team, Stan Lee and Jack Kirby upended the status quo of the team in #16, almost completely changing the roster of the title. In the issue, all of the remaining founding members decided to all step down, to focus on their solo adventures and private lives. Although he also had his own solo adventures as a co-feature in Tales of Suspense, Captain America remained and reformed the team with new recruits. The first was Hawkeye, a former reluctant adversary to Iron Man. The other two were Quicksilver and Scarlet Witch, who as members of Magneto’s Brotherhood of Evil Mutants were adversaries to the X-Men. The introduction of these three characters was the earliest example of former villains living reformed lives as heroes.
In the story, the departure of the old Avengers and the introduction of the new was held at a press 
conference, an event which would be replicated throughout the series when significant roster changed occurred. In the original issue, the press members showed mostly unquestioning enthusiasm, but in Avengers #1.1 from 2016, the press showed a much stronger skepticism towards replacing the former symbols of virtue with known criminals. These five 2016 issues numbered 1.1 through 5.1 detailed a never-before-seen tale of the woman Cressida, dubbed Avenger X. She was discovered by the new team of Avengers and invited to join but deceived the team about the fatal repercussions of her powers. After she fought the team, she was presumed escaped until the 2016 Avengers team rediscovered her and trapped her in the Microverse.
This new team of Avengers certainly seemed smaller, under-powered and less cohesive than the original roster… and this was not helped when their next recruit, the Swordsman, turned out to be a traitor. He attempted to join in issue #19 but was exposed and driven off soon after. In issue #20, the villain the Mandarin tricked the Avengers into believing that Iron Man had ordered 
Swordsman to join the team. In the very short time that he had membership, Swordsman had a change of heart and secretly helped thwart the plans of the Mandarin, before being driven off by his new team in the same issue he joined. He would return several times as an antagonist until years later he repented and officially rejoined in issue #114.
The first of the original founding Avengers to return to the team were Giant-Man and the Wasp. Reappearing only 10 issues after they left with #26, the story of their return occurred over several issues. Beginning with a distress signal from the Wasp, the Avengers investigated and were ultimately reunited with Hank Pym, the Wasp’s partner and former Giant-Man. Taking on a new identity of Goliath, Pym aided the Avengers is rescuing the Wasp from the Collector in #28. Unfortunately, it turned out that Goliath’s size-changing powers had been adversely affecting his body and by the end of the story his powers had shorted out, leaving Goliath stuck at a permanent height of ten feet. This would be Goliath’s status quo for some time.
Following Wasp & Goliath’s return, the Avengers’ roster remained fairly stable for the next several years of publication. Starting in issue #35, former villain and love interest of Hawkeye, the Black 
Widow, began unofficially assisting the team on missions. Hawkeye would push for full membership for her but she decided to retire from crime-fighting before a decision would be reached to induct her in issue #45. Issue #38 saw the Olympian god Hercules join the adventures of the Avengers as a powerhouse replacement for his friend, the Norse god of thunder Thor. Hercules and Black Widow would both serve as unofficial members for the next several missions. Issue #45 offered membership to Hercules, which he accepted in the next issue. However, this expanded roster would not last long and the Avengers would soon find themselves short staffed yet again.