Heres how I have allowed multiple users to be able to edit one node. Custom node_access module

hook__node_access_records()
 // add the admin access row
    $grants[] = array(
      'realm' => 'og_admin',
      'gid' => $node->group,
      'grant_view' => 1,
      'grant_update' => 1,
      'grant_delete' => 1
    );
    return $grants;

then..
hook_node_grants()
foreach ($account->og_groups as $group) {
      if ($group['is_admin'] == 1) {
        // grant this user admin priv over all nodes in group
        $grants['og_admin'][] = $group['nid'];
      }
}
return $grants

Then hook_enable()
  node_access_rebuild();

Its simple and it works. I may have some typos in there but thats the basic idea. Same trick can be applied to roles. So you can have editor roles that can change any node on the site.
-- 
Scott Reynolds
Senior Developer and Engineer
MothersClick
scott@mothersclick.com