Well, there was the "Council of Reeds" from Jonathan Hickman's Fantastic Four
run, an assembly of the various incarnations of Reed Richards from across the multiverse, but I think they turned out to be more villains than heroes. (EDIT: Oops, beat me to it.)
And I can think of two times it's been done in animation: in the concluding "Spider Wars" storyline of the '90s Spider-Man
animated series, a team of alternate-timeline versions of Spidey/Peter were brought together to save the multiverse, and in "Game Over for Owlman!" on Batman: The Brave and the Bold
, Batman defeated the evil-twin mirror-universe counterparts of the Justice League by recruiting a team of alternate Batmen from multiple realities. The video game Spider-Man: Shattered Dimensions
also has a team of multiple Spideys from four different timelines/eras.
I'm not sure if this quite counts, but there were a number of Wonder Woman
comics from the '40s or '50s that told "imaginary stories" teaming Wonder Woman up with her younger selves Wonder Girl and Wonder Tot -- which led to some confusion when the writer of a different comic thought Wonder Girl was a separate person from Wonder Woman and started writing her into present-day, canonical stories. So they had to retcon her into a distinct character and it got kind of messy from there.