But the phrase doesn't say they are in a house. They could be in a garden at the top of a hill, for instance. Besides which, I know I've used the term 'below' to mean downstairs before. 'the guests are below' is definitely a term I would use. I wouldn't say 'below' is a wrong answer just because downstairs is more usual.
Just to be clear, in this context we are using "to wait for" correct? Does that make the preposition "en"? If "awaiting" was used, would you omit the en but the construction would otherwise remain the same? French prepositions man..... I dunno.
Side note: Is it "attend" and not "attendons" because the guest is the subject of the sentence?
"en bas" has no link with verbs "attendre/await", that are directly transitive, i.e. there is no need for a preposition, unlike "wait for", to introduce its object (nous/us):
- our guests are awaiting us downstairs = our guests are waiting for us downstairs = nos invités nous attendent en bas.