Translation:He attended a conference in our office.
According to the definition in NPIV, "konferenco" really means "conference," in the sense of a large gathering of (usually) professionals, presumably in a large space (like a conference center). Such a gathering could not occur in an office. Maybe "kunveno" would be more appropriate.
I rarely say "attend a conference" It's usually "at a conference" & ĉeesti = ĉe esti = to be at.
Yes, I did report this.
It may or may not be unusual usage of language, but either way I think it's going too far to say that it's wrong. "Ĉeesti" clearly is "be at" etymologically, but as a translation I'd say "attend" is also spot on.
I didn't say that "attend" is wrong, just that it's not the only expression which can fit here.