No-one likes to be on the outside looking in.
One of the easiest ways to keep employees engaged is to make sure that they're included properly. That means ensuring that they have access to the information they need to do their jobs, and to understand anything else that might affect them as quickly as anyone else in the organization. This often boils down to effective dissemination either via e-mail, communal message boards or formal memos.
Action step: If you're not 100% positive that this is true for your employees, why not take a little time to review your internal communication systems to make it so?