Man, I just got off my roof, after tearing off a ton of shingles and doing some sheeting repair.
Yeah, you can stand on your roof but it is a bad idea. No handrails or proper access, and usually some beer is involved on the 4th.
That could ruin somebody's day, or life.
Edit: a flat roof, and will you be climbing out a window?
You can have a few people up there, just string a line across about 5 feet back of the edge to keep people back. I've seen some terrible fall accidents at the workplace, would not wish that on anybody.