Alert after form radio has been selected
'wrapper' is a required key for #ajax on form elements, you're probably just missing that. From the docs


  This property is required for using AJAX requests in on a form element


If you add the wrapper it should work:

$form['field_radio']['#ajax'] = array (
   'callback' =&gt; '_get_member',
   'progress' =&gt; array('type' =&gt; 'throbber', 'message' =&gt; ''),
   'wrapper' =&gt; 'some-element-id'
);