How do you push?

If any object can be interacted, you usually just walk over to it until the interaction prompt appears, then hit space.

Assuming you mean the pushing the dumpster, walk over to the left side of it (there should be an interaction circle indicating the spot) and press space to interact. Then wait for one of the friends to join you. Use the D (or right arrow) key to push. Oddly, it appears to be the only time in the game where you have an extra step for interacting with things.

I never saw an instruction to use the space bar. The game is much easier now.