Return of Reckoning

Can someone please ELI5 how guard is supposed to work, particularly in scenarios?

When I queue solo, I typically join party 1 and am able to guard and guard switch as needed. When I join while already in a party, and everyone in that party has pre-coordinated which scenario party to join, I can guard nobody--even though we are all in the same scenario party. I don't understand.

And as an aside, is guard being limited to your party working as intended, or is it some unfortunate externality from the adjustment made to group heal. Unless I'm missing something, there doesn't seem to be an epidemic of tanks intent on surveying the battlefield ready to switch guard at a moments notice. If anything, it seems like a lot of us have to be reminded there is a guard button at all. Making the execution of one of our core functions more difficult seems like it will incentive tanks in the wrong direction--giving up on trying to be a reactive guarder altogether.

I dunno, maybe I'm just seeing it wrong or need to L2P or something. Thoughts?

This is a problem with scenario groups not working terribly well, and the guard buff not updating properly. Also, there might be an issue with joining a scenario group after the match starts as opposed to when you first load in. When I use guard, I'll frequently get a message that this player is not in my group, but hit guard a second time and it'll apply if they actually are.