A crowd of people had formed in front of the station.
On summer nights, you can see swarms of insects around the light.
群がる is the verb form meaning 'to crowd around' or 'to swarm'. 群がり is the noun derived from it, referring to the resulting crowd or the act of swarming.
Standard kanji spelling for this noun form.
Rarely used kanji form; 叢 is an uncommon character for 'cluster' or 'thicket'.
Rarely used kanji form; 簇 is an uncommon character for 'swarm' or 'crowd'.
Noun form of the verb 群がる (mura-garu), which is derived from 群 (mura, 'group') + suffix がる (garu, 'to seem, to be inclined to'). The kanji 群 means 'crowd' or 'group'.