Use proper hooks

As documented in `private/group.php`, this is a `OC_Group` hook and not `OC_User`…

```php
/**
 * This class provides all methods needed for managing groups.
 *
 * Hooks provided:
 *   pre_createGroup(&run, gid)
 *   post_createGroup(gid)
 *   pre_deleteGroup(&run, gid)
 *   post_deleteGroup(gid)
 *   pre_addToGroup(&run, uid, gid)
 *   post_addToGroup(uid, gid)
 *   pre_removeFromGroup(&run, uid, gid)
 *   post_removeFromGroup(uid, gid)
 */
```