crowd, transliteration of Sanskrit "kSo"
閦 centers on a crowd or multitude of people. The Sanskrit transliteration sense is a specialized use of the same character for the sound "kSo".
閦 combines 門 (gate) with 众 (crowd), suggesting a crowd gathered at a gate. The character was likely created to represent the concept of a multitude.
Picture a gate 門 with a crowd of people 众 pressing through it. The gate frames the crowd, making 閦 a vivid image of a multitude.
For シュク, imagine a shoe queue forming at the gate: shoe queue -> シュク, as the crowd waits to enter.