The “core group” I normally think of when I think of the Avengers is Captain America, Iron Man, Thor, Hulk, Wasp, Ant Man/Goliath, Hawkeye and the Vision.
Basically, if you’re a hero in the Marvel Universe and aren’t actively affiliated with another super-group, you probably have an Avengers card, and sometimes you have one anyway. Just off the top of my head, USAgent, Black Knight, Beast, Wonder Man, Luke Cage(Power Man), Iron Fist, Black Panther, Spider-Man, Spider-Woman, Scarlet Witch, Quicksilver, Black Widow, She-Hulk, Wolverine and Hercules have all been associated with the Avengers at one time or another, and I know that’s not all of them.