There was a flock of pigeons in the park.
A group of tourists was walking behind the guide.
wide expanse (of flowers)
Used for a broad, visually striking spread of flowers, often in natural settings. Less common than sense 1.
A wide expanse of rapeseed flowers spread across the hillside.
一軍 (いちぐん) means an army or a top sports team, while 一群 (いちぐん) is an unrelated word meaning a cluster or group of things, despite sharing the same reading.
flock of sheep
Compound of 一 (one) and 群 (group, crowd). The reading いちぐん follows standard on'yomi. The sense 'wide expanse of flowers' is a figurative extension of the group meaning.