Never taken 600ppl before, but at that size and from seeing such corporate photos with many people, these are 'not important photos' in the sense that most likely they will/can never be viewed under too much scrutiny
Most people won't bother looking too long at them either for various reasons (eg. Not printed too big; can not tell too clearly who is who; cannot/hard find who is who; on corporate website or publications where readers can't be bothered with such photos (just turn to the performance summary page), etc)
So don't loose hair over this shot.
Maybe at least get the important folks (who are paying you) in the front and in good focus.
Other than that, as suggested, take it as any other shot or like a landscape shot.
My worthless few cents....