Using More Tag in Combination with the_excerpt

first of all you don’t need to echo the_content(); and the_excerpt(); both methods already echod, second simply put the more link using permalink after the_excerpt(); like this.

<?php // IN X USER GROUP
    if($customgroup==1 || // Are they in the custom group?
        $userinfo->usergroup == 66) { // or the admin of the website?
        the_content();
    } else { // They aren't in the custom group 
        global $more;
        $more = 0;
        the_content();
    } 
?>

Edited: added second param true to the_content('', true) this will cut off the content after <!--more--> quicktag.