General synonyms:
* Plot: To secretly plan or scheme, often for something illegal or harmful.
* Scheme: To plan or devise a plan, often a cunning or underhanded one.
* Collude: To work together secretly, often for an illegal or dishonest purpose.
* Intrigue: To plot or scheme secretly, often using cunning or deception.
* Connive: To secretly cooperate in wrongdoing or fraud.
* Machinate: To plot or scheme, especially to achieve power or gain.
More specific synonyms:
* Consort: To associate with someone for a shared purpose, especially a harmful or criminal one.
* Cabal: A secret group of people who plot or scheme.
* League: A group of people who are united by a common interest, especially one that is secret or illicit.
* Clique: A small, exclusive group of people who have a common interest or purpose, often with a secretive or conspiratorial aspect.
* Syndicate: A group of people or organizations who work together to achieve a common goal, often illegally or immorally.
The best word to use will depend on the context of your writing.