It's difficult to say from those pictures, but the question I have is were they really nesting in the ground or just working there? 70% of bees nest in the ground, and while almost all of those are solitary, many nest aggregately, meaning they'll build their nests near each other, but they don't work together to build the nests, which could explain why so many were "bunched" together. Mason bees actually do not nest in the ground, they nest in holes drilled by other insects in wood or in hollow stems, but they gather mud to cap their nest chambers with. If they are mason bees, then they weren't nesting in the ground, just collecting mud there, and many females could easily be working the same patch of dirt.