group, crowd, multitude, mob
羣 centers on a gathered collection of living beings: a group, crowd, multitude, or mob. The idea is many individuals together in one place.
羣 is an older form of 群. It combines 君 (which likely contributes the sound) with 羊 (sheep), suggesting a flock or herd of sheep as a representation of a group.
A lord (君) oversees his flock of sheep (羊) gathered together: that's a group or crowd.
For グン, imagine a huge group of goons (グン) crowding around the lord and his sheep.