To me, the statement "two or more individuals working together" means that it's a collaborative effort in order to conduct the suspicious activity.
For example, my institution checked off that box in a scenario in which two or more individuals were being used to structure cash out of an account. In this particular scenario, 2 of the individuals were signers, and 1 was not, although I don't believe that would have made a difference, as regardless of their relationship to the account they were still working together.
I don't have it in front of me currently, but it's always helpful to hover over the box in question on the e-filing site, as it brings up additional information, although sometimes that does lead to further confusion!
Life is like a bicycle. To keep your balance, you must keep moving.