From the Model Mayhem FAQ:

What is a Proactive Alert?
A Proactive Alert is a special private message you activate with a Sponsored Casting Call. These messages can be sent to up to 20 members of your choosing. These alerts proactively inform members about the casting call associated with the alerts.

You activate Proactive Alerts with a checkbox during the Sponsored Casting Call creation process. You can also add them later if you choose.