for them, you would say 뭐 하세요? the 세 (시) is what adds the honorifics. (by the way, it's mwo, not bo, and se-yo, not seo)

if you were talking to someone of your own age or of equal rank, you could just say (name)씨, 뭐 해요? which is the polite form. unless you are just meeting the person (introductions). normally then, you would still use honorifics.

if you were speaking to close friends and other people you have closer/intimate relationship with, you would drop the 요all together and simply say 뭐 해? which is the intimate speech style.