Use this code in the address bar to view the user_id of each user in your wpmu user list

javascript:var user_id = jQuery('span.edit a').each(function(){ var sHref = jQuery(this).attr('href'); var sStart = sHref.indexOf('='); var sEnd = sHref.indexOf('&'); jQuery(this).parents('div.row-actions').before(sHref.slice(sStart +1,sEnd));});

It’ll look like this
showuserid