So who is, has been, can be, or will be a member of Earth’s Mightiest Heroes?
Most writers agree that The Avengers is the “Dream Team” of Marvel’s groups. Membership is special, something to be earned and respected. This is reflected in the pomp and ceremony of many membership announcements or invitations made throughout the series. In the early days these were often associated with public announcements, press conferences, and acceptance speeches.
However at times, members do seem to have been admitted “at the drop of a hat” and this has to some extent lessened the honor and status of Avengers membership. With well over 50 members including constituents of the Fantastic Four, the Inhumans, the Eternals, the Defenders, the X-Men, the Thunderbolts, and the Champions it may seem to casual readers that all the characters in the marvel universe belong to the Avengers.
The Avengers operate a formal and structured membership process. Within days of deciding to form a team, the group had developed a written charter and a set of by-laws. Section two of the by-laws deals with membership in detail. It was many years before readers actually saw the contents of these by-laws and we should expect that this version is significantly amended from the original drawn up after Issue #1. We can assume however that the original set still contained many basic democratic and human rights provisions. It is also some time since readers last saw a working version of the by-laws and events both on-panel and behind the scenes present clear evidence that changes to the charter must have continued to take place over time.

As time proceeds, the Avengers membership changes with their charter even stating: “membership of the Avengers may be subject to, and, indeed might profit from change”. Accordingly they develop clear rules and procedures to govern Avengers membership and admission.