noun, used as a suffix, noun, used as a prefix, noun
Used as a suffix, prefix, or standalone noun to indicate a cluster or gathering of similar things, often plants, clouds, or abstract qualities. As a suffix, it attaches to nouns to mean 'a group of ...'.
雲が群をなして流れている。
Clouds are drifting in clusters.
このあたりには杉の群が点在している。
Clusters of cedar trees are scattered around this area.
彼は才能の群だ。
He is a bundle of talent.
群れ is a more common standalone noun for a group or flock of animals/people, while 群 is often used as a suffix/prefix or in literary contexts for clusters of inanimate things.
集まり is a general noun for a gathering or meeting of people, whereas 群 emphasizes a natural cluster or mass, often of non-human things.
The reading むら is native Japanese, related to 群れ (むれ) and 村 (むら), all sharing a sense of 'gathering' or 'cluster'. The exact historical derivation is uncertain.