customize welcome email subject

I want to change the default "New ___ User: ___" subject line in emails that come after signup.

The SB Welcome Email plugin isn't working as it doesn't disable the default message. Seems like lots of people are having this problem but the plugin developer isn't responding yet.

I saw some suggestion about adding code to the functions file, but that didn't work for me either. Here's the modified code I used:

function change_activation_subject( $subject ) {
		$subject = __( 'Welcome to cre8tives' );
		return $subject;
	}
	add_filter( "wpmu_signup_blog_notification_subject", "change_activation_content" );