to assemble. to gather together
亼 centers on bringing things or people together: assembling or gathering into one place.
亼 is an ancient character whose exact historical development is uncertain. It is traditionally associated with the idea of gathering or assembling, but the component origin is not clearly established.
The shape looks like a simple roof or cover over a flat surface, as if people are gathering under one shelter.
For シュウ, imagine a shoe gathering dust under the roof: shoe -> シュウ, and the shoe collects dust as people assemble.