Shared Solution: Customizing Activation Email

Just wanted to share my solution to this topic http://www.gravityhelp.com/forums/topic/customizing-activation-pageoutput?replies=8#post-102243

This code goes into your theme's functions.php file.

Changing the subject was the straightforward part:

add_filter( 'wpmu_signup_user_notification_subject', 'filter_subject_and_content', 10, 4 );
function filter_subject_and_content( $text ) {
    return 'Welcome to the site!';
}

To change the message I essentially combined the gf user registration filter with the core code so I could change the message without disrupting the process:

add_filter('wpmu_signup_user_notification_email', 'modify_signup_user_notification_message_again', 10, 4);
function modify_signup_user_notification_message_again($message, $user, $user_email, $key) {
	$message = sprintf(__(( "To activate your new account, please click the following link:\n\n%s\n\n After you activate, you can change your password to something you can actually remember.\n\n" ),
			$user, $user_email, $key, $meta),site_url( "?page=gf_activation&key=$key" ));

        return sprintf($message);
    }

You can change that text to anything you want instead of the vanilla wp text/email.