collect, gather, assemble
萃 centers on bringing things together into one place: collecting, gathering, or assembling people or items.
萃 combines 艹 (grass) with 卒, which likely contributes the sound and suggests a group of soldiers, giving the idea of gathering like grass or troops assembling.
Grass 艹 on top of 卒 (soldiers) looks like soldiers gathering in a grassy field. Picture troops assembling among the grass.
For スイ, imagine a swift gathering of people: swift -> スイ, and everyone collects quickly like grass bending in the wind.