group, crowd, swarm
簇 centers on a dense gathering of living things: a group, crowd, or swarm. The idea is many individuals clustering together.
簇 combines 竹 (bamboo) with 族, which likely contributes the sound and the idea of clustering. The character suggests bamboo growing in dense clumps, extending to any crowded gathering.
Bamboo (竹) often grows in thick clusters, and 族 (tribe, family) reinforces the idea of many individuals together. Picture a dense bamboo grove where every stalk is part of a larger crowd.
For ゾク, imagine a zookeeper trying to manage a swarm of animals clustering around the bamboo: zoo -> ゾク, and the crowd of creatures matches the kanji's meaning.