With the flood of charicters that could claim to be Avengers after Civil War, is Marvel Inc. replacing the old term Marvel super-heroes with Avengers for the Non-X men titles etc.
Personally I love the continuity of a team that has survived so long adding members by drip-feed. You can pretty much cut it any way you like - its in the eye of the reader after all. For me, if someone hasn't been offered / accepted membership in the Mighty Avengers they are not an Avenger. Wolverine is not an Avenger. There are no doubt exceptions: Dman got the offer in Cap's book.
Remember when a whole bunch of mutants (and at least one X-title) got crucified by Austen? Some of the mutants were poorly drawn shapes, nameless and characterless. If we have to accepts the 'guys at the back' as Avengers something special will have been thrown away.