Hook_node_grants: how do I access a user account field?
As per your error statement, field_group does not exist in $user object. The global $user object does not load the fields which are attached to user profile. if you want to load the complete user profile with attached fields, you need to use function "user_load" in your code.

global $user;

$account = user_load($user-&gt;uid);
if ($account-&gt;field_group[LANGUAGE_NONE][0]['value'] == $node-&gt;group &amp;&amp; $op == 'view') {
  //Do the needful
}