A TF who is about to start an operation, but still has player spots open, can broadcast a signal to call for aid. Players who like to do extra operation attacks can subscribe to this feature in the menu. When a TF sends a call for help, the player receives an invite in their BB mailbox. If the player accepts, the TF can accept/decline (they see the players rating *). Upon acceptance, the TF gets filled virtually with these helper players and can start the op when they wish.
Maybe needs some time limits or other constraints so TF leaderboard competition stays fair.
* TF can rate the helpers attack with review stars so helper players who often like to mess up their attack on purpose can be identified/ignored.