How can I get a user by email address using a service?
Apparently Params are only for index functions. So you need to implement an index function, something like this from the user_resources.inc file in the Services module:

'index' =&gt; array(
    'help' =&gt; 'List all users',
    'file' =&gt; array('type' =&gt; 'inc', 'module' =&gt; 'services', 'name' =&gt; 'resources/user_resource'),
      'callback' =&gt; '_user_resource_index',
      'args' =&gt; array(
        array(
          'name' =&gt; 'page',
          'optional' =&gt; TRUE,
          'type' =&gt; 'int',
          'description' =&gt; 'The zero-based index of the page to get, defaults to 0.',
          'default value' =&gt; 0,
          'source' =&gt; array('param' =&gt; 'page'),
        ),
        array(
          'name' =&gt; 'fields',
          'optional' =&gt; TRUE,
          'type' =&gt; 'string',
          'description' =&gt; 'The fields to get.',
          'default value' =&gt; '*',
          'source' =&gt; array('param' =&gt; 'fields'),
        ),
        array(
          'name' =&gt; 'parameters',
          'optional' =&gt; TRUE,
          'type' =&gt; 'array',
          'description' =&gt; 'Parameters',
          'default value' =&gt; array(),
          'source' =&gt; array('param' =&gt; 'parameters'),
        ),
        array(
          'name' =&gt; 'pagesize',
          'optional' =&gt; TRUE,
          'type' =&gt; 'int',
          'description' =&gt; 'Number of records to get per page.',
          'default value' =&gt; variable_get('services_user_index_page_size', 20),
          'source' =&gt; array('param' =&gt; 'pagesize'),
        ),
        array(
          'name' =&gt; 'options',
          'optional' =&gt; TRUE,
          'type' =&gt; 'array',
          'description' =&gt; 'Additional query options.',
          'default value' =&gt; array(
            'orderby' =&gt; array(
              'created' =&gt; 'DESC'
            )
          ),
          'source' =&gt; array('param' =&gt; 'options'),
        ),
      ),
      'access arguments' =&gt; array('access user profiles'),
      'access arguments append' =&gt; FALSE,
    ),
  ),