Teaming up with someone's team doesn't make you a team member. Do you really think the X-Men or Magneto himself considered himself an X-Men member when they were working together in X-2 lol? Spider-Man has teamed with the X-Men before but that didn't make him one of them.
As for First Class, they were not the X-Men yet.
Not letting that one go huh ? Wasn't it you who initially said that this is about the movies (in response to my comic book examples) ? So when did Spidey team up with the X Men hmmmm.....
So if they weren't the Xmen in a film called "Xmen first class" who were they ? It wasn't called "The mutants who would later become the Xmen, but aren't the X Men yet, guest starring the guy who will later become Magneto"
He even wears an X-uniform for a while.
What about DOFP ? Future Magneto works alongside the Xmen pretty solidly (even when past Magneto was treacherous).
Really though does this matter ? If it makes you feel better I'm happy to agree that Magneto is not a superhero - and given how Magneto sees the world, from his perspective its not like he's working with the Xmen, it's more like they were working with him.
Now back to what this thread is really about, what about ........
the Guardians of the Galaxy working together to control the infinity stone
and disintigrate Ronan
Johnny Storm combining all the powers of the Fantastic four to take out Dr. Doom.
and this....
Spiderman saving the rail car full of kids and MJ from the Goblin was pretty impressive.
Pretty impressive, a double save, both the kids and MJ all at once - although you have to give the assist to the great people of NYC.