I think, if you mean someone shares with you a background character’s visual design only (like a specific appearance combo), then that’s okay. But if you mean someone shares with you a character they already developed and you keep them the same and then write them into your story, then that’s when it breaks the guidelines. So if my friend’s story has the protagonist meet a guy named Sam, and then in my story Sam shows up again, that would be against the rules.
TLDR; If a DC movie had a scene where a guy that looks like Tony Stark walks by in the background, that just happens, but if a DC movie had a scene where it’s explicitly Iron Man, that’s a crossover.