Except for narcotics conspiracies (Title 21), plain old Title 18 conspiracies require an act "in furtherance" of the conspiracy by ONE of the conspirators. Buying a map of the area around the bank your team plans to rob is sufficient.

(A drug conspiracy requires no "act in furtherance.")

So if you are "just joking" with your team and one of them takes your joke seriously and purchases bank robbery supplies or puts an SD card into a sandwich you are liable.