Turning page titles to links using hook_page_alter()
I don't think hook_page_alter() is the best place to do this (though I may be wrong), it seems like since you're changing the display of the title it should be done at the theme level.

Personally I'd go for a hook_preprocess_page():

function MYMODULE_preprocess_page(&amp;$vars) {
  if ($node = menu_get_object()) {
    $field_items = field_get_items('node', $node, 'field_user_id');
    $first = array_shift($field_items);

    $user_id = $first['value']; // if 'value' is the column name for your field, it might be 'uid' if you're using a references field, 'target_id' for entity reference, etc.

    $link = l($vars['title'], 'path/for/link');

    $vars['title'] = $link;
  }
}