Registered users belong to the group with id 2. I tried to find group names constant somewhere, but I failed when I was researching for my project, so I ended up making my own constants.
Anyway, to get users who belong to a group, you can use
$registeredUsers = JAccess::getUsersByGroup(2); // in my project it was $self::REGISTERED_GROUP
Add
jimport('joomla.access.access');
jimport('joomla.user.user');
before if neccessary classes are not loaded.
Then you could do
foreach($registeredUsers as $user_id) {
$user = JFactory::getUser($user_id);
echo $user->name;
}
Important: Make sure before selecting users using the id
passed in the link that it belongs to id group list which you want exposed. I guess you wouldn't want to expose super administrator usernames etc