KanjiJLPT N2
Flock

flock, group, crowd, herd, swarm, cluster

Mnemonic

Mister Henderson is the wealthiest sheep rancher in the county. Every dawn he steps onto his porch and whistles—hundreds of woolly sheep come stampeding over the hills toward him, forming one enormous flock that blankets the entire valley under the mister's watchful eye.
Additional thoughts
Picture a distinguished gentleman on a porch, sheep flooding toward him like a white tidal wave. The mister owns the sheep; the sheep form the flock. Ownership plus animals equals a gathered crowd.
Quick recall
A proud mister whistles and all his sheep rush together into one mighty flock.

Details

The keyword for 群 is flock. This kanji refers to a group or gathering of animals, people, or things that come together in large numbers. It conveys the idea of a crowd, herd, or cluster, emphasizing the collective nature of many individuals assembled in one place. It can also be used more abstractly to describe a multitude or a swarm of anything grouped together.
On'yomi
ぐん
Kun'yomi
む.れる、む.れ、むら、むら.がる