Fatal error: Call to undefined function wpmu_validate_user_signup()

Hi there!

Just installed Add Users plugin and I am receiving this error when attempting to add a new user.

Fatal error: Call to undefined function wpmu_validate_user_signup() in /home/boldmediagroup/onemultisport.com/wp-content/plugins/add-new-users/add-new-users.php on line 256

Using WP 3.0.5
I am a tad green so please be gentle.

Thanks in advance,
Rob