How to identify the logged in user inside a plugin?
I'm a bit confused by your question.

It sounds as though you want to check if the currently logged-in user is an Administrator, but it looks like you're actually checking whether the newly-registered user is an Administrator.

Here's how to retrieve the currently logged-in User, and check whether they belong to a specific User Group.

function (Event $event)
{
    $user = craft()-&gt;userSession-&gt;getUser();

    if ($user &amp;&amp; $user-&gt;isInGroup('groupHandle') {
        // Do stuff.
    }
}


Alternatively, you could check whether the current user has permission to perform a specific action:

if ($user &amp;&amp; craft()-&gt;userPermissions-&gt;doesUserHavePermission($user-&gt;id, 'registerUsers')) {
    // Do stuff.
}