"Nos" in this sentence simply emphasises the fact that it is "us" who would have drank all the beer. If you omit it in this sentence, the meaning won't change but it won't have that emphasis. We make some verbs "pronominal" (i.e add the reflexive pronouns) to have this nuance.
"Beber" means "to drink", but "beberse" means "to drink up". So "nos habremos bebido la cerveza" emphasizes the fact that we will have drunk all the beer up!
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