The most reasonable way to ensure compliance is to program the boarding software to beep loudly and embarrass the person trying to board when GS or 1K or any other pre-boarding group is called and the person isn't in those groups. It happens when BG2 tries to board when BG1 is boarding, etc. No reason they can't do it for GS/1K at least.

And if you're boarding with someone who isn't in your group you just let the GA know and they'll override the "beep" and allow them to board, too. Simple IMO.

-RM