Iron Man, War Machine, Spider-Man, Rocket, Groot, Hulk, and others are going to be mostly CGI creations during fight scenes.
While there is surely going to be a scene in one of the next two Avengers films that has every Marvel hero together fighting with one another, that does not mean that they will all be there filming that scene together.
That said, it is not uncommon for actors in these larger films to go throughout an entire production without meeting some of their co-stars.
Hopefully this is not the case and thanks to careful and meticulous planning on their part, Marvel has figured out how to make this happen.