Arriver can mean to happen. With that meaning, it’s a member of a class of verb that always a take a dummy subject pronoun and have their “real” logical subject expressed as a dative indirect object. See also falloir (il nous faut nous dépécher = we must hurry), aller in the sense of suit (ça me va totalement = suits me fine) or the archaic verb chaloir (peu m’en chaut = I care little about this).

You could translate “il nous arrive à tous de dire” as “it happens to us all to say” or more idiomatically “we all sometimes say”. The inversion of il is purely stylistic, and triggered by the adverb certainement.