Nouns

(n)confederate, collaborator, partner in crimesomeone who assists in a plot
(n)accomplice, confederatea person who joins with another in carrying out some plan (especially an unethical or illegal plan)

Verbs

(v)band together, confederateform a group or unite“The groups banded together”
(v)confederateform a confederation with; of nations

Adjectives

(s)allied, confederate, confederativeunited in a confederacy or league