gather together
蔟 centers on bringing things together into one place: gathering, assembling, or collecting.
蔟 combines the grass radical 艹 with 族, which likely contributes the sound and the idea of a group or cluster, suggesting plants growing together.
Grass 艹 growing in a tight cluster 族: imagine many blades of grass gathered together in one spot.
For ゾク, picture a zoku (族, tribe) gathering together under the grass: zoku -> ゾク, and the tribe clusters like plants.