To crowd, flock, or move in a group; to be in a herd or cluster (intransitive). Often used for animals or people acting as a group.
冬になると鳥が群れて南へ渡る。
In winter, birds gather in flocks and migrate south.
若者たちが駅前に群れて話していた。
Young people were gathered around the station talking.
河原で小魚が群れて泳いでいるのを見た。
I saw small fish swimming together in a school at the riverbank.