If you say "Kan du gå längre bak så att man kan stiga på bussen?", then you talk only to one person ("du"). To speak to several people, you must use "ni" instead. "man" is a bit unpolite to use in this context (if the driver is service minded), it sounds better with "fler" = "more" (as in "more people").
"Kan ni gå längre bak så att fler kan stiga på bussen?"