A crowd of people looking up at 3 people on the edge of the roof of a building.
The crowd on the ground is watching 3 people on the roof's edge.
Looking up at is the same as watching, and the edge of the roof is also called the roof's edge.
3 people on the roof's edge is rephrasing "3 people on the edge of the roof of a building".
The crowd is on the ground if they are looking up at the people on the building.