This we're seeing while using the new OG sync code. When a user A subscribes to an organic group X, A is added to X as a pending member. If B has admin rights over X via an ACL, B can see A only when A is actually "Added" to the group. B can not see A when A is "pending'.
Under these circumstances, B cannot have access to the CiviCRM record for A to make the decision of whether to approve A. This is a severe problem in using OG with CiviCRM.