Collusion always involves secrecy. It's not done openly, as it involves deceit or trickery. The employees were in collusion to embezzle funds, holding secret meetings to plan.
Pro Tip 2/3
Negative Connotations
Collusion suggests dishonesty and is used in contexts where trust is betrayed. The two firms were in collusion to fix prices, which is unfair to customers.
Pro Tip 3/3
Not Always Illegal
Collusion is not always against the law but is generally viewed as unethical or unfair. Their collusion to exclude her from meetings wasn't illegal, but it was morally questionable.