WordNet sense 1 ((chemistry) two or more atoms bound together as a single unit and forming part of a molecule):

radical, chemical group

WordNet sense 2 (a set that is closed, associative, has an identity element and every element has an inverse):

mathematical group

WordNet sense 3 (form a group or group together):

aggroup

WordNet sense 4 (any number of entities (members) considered as a unit):

grouping