WordNet sense 1 (someone who assists in a plot):

collaborator, henchman, partner in crime

WordNet sense 2 (a person who joins with another in carrying out some plan (especially an unethical or illegal plan)):

accomplice

WordNet sense 3 (united in a confederacy or league):

allied, confederative

WordNet sense 4 (form a group or unite):

band together

WordNet sense 5 (in or characteristic of a region of the United States south of (approximately) the Mason-Dixon line):

southern