Communal nesting bees
Although mining bees are solitary, a handful of species in the UK often nest communally. Communal bees have a simple form of social structure: they share a nest, but individual females will build their own cells, lay eggs and provision their own offspring: they do not have the complex social organisation with workers and division of labour of honeybees.
If a high level of relatedness is not responsible for the evolution of communality, why do these bees nest communally? A potential benefit is improved nest defence. There are always bees going in or out, and in my own observations, a few bees often sit by the nest entrance (like as in the photo at the top), therefore opportunities for parasitism by cleptoparasites might be reduced.