Four of the seven are. Loki would be very insulted if you called him an Avenger, I think. He's the one who brought the Chitauri in the first place, though it's possible that act wasn't entirely of his own free will. Agent Carter hails from the 1940s; she founded the parent organization that would later guide the Avengers.
video;
[AND THEN THERE'S WINTER.]
...The last is a bit more complex.