It could be that some of these characters will just appear in the background on TV news reports, say. Or there could be one brief scene where Superman helps a team of them out of a jam, before the story refocuses just on Superman and his immediate cast. Just enough to establish, as theenglish said, that this is a world where multiple heroes exist, without the story actually being about them.
Think about how the first Shazam! movie engaged with the wider DCEU. The other heroes were there, the characters were aware of and influenced by their existence, but they were incidental to the story being told. They were just part of the background texture of the world.
Or, heck, since it's a James Gunn movie, maybe the best analogy is how Gunn handled the cameos of several big-name stars as Reavers in Guardians 2. They were basically just there in one scene, IIRC, enough to establish them without taking up a significant piece of the story.