The Defenders is the greatest non-team team in the Marvel Universe. There is no core membership, no kind of official organization or charter, or even a health plan. There are some recurring players in different iterations of the group but it gets used like a public domain hero name. Whenever there’s a crisis and a group of supers is around for it, they get to be the Defenders; then everyone goes home.
References: [marvel.fandom.com: Defenders]
[org]
Metahuman Database 