RunSignUp allows you to set up protected registration, which will let you create a password that all registrants will be required to enter in order to sign up.
Setup Password Protected Registration
To setup a password protected registration, you can select Race > Display > Protected Registration
Begin by clicking on the Add Another Password button.
You can then set a Start Date and End Date for when this particular password is in effect
You can then set a Password and Confirm that Password.
You can use the Add Another Password to add additional passwords for additional periods.
To remove a protected registration window, simply click the X in the corner of the setup box, and Save Settings.