Display A list of Admins

Use get_users function it allows you to query user by role.

For contributors,

 $contributors = get_users('role=contributors');
foreach($contributors as $contributors){
   // do something with contributors
}

For Admins,

$admins = get_users('role=admin');
   foreach($admins as $admins){
       // do something with admins
    }

replace role=* with your desired roles.