Fixingextrreme dynamic range issues in crowd shot

I was asked to take a photo today and the group leader insisted I take it against a hopelssly bright background, so the people are properly exposured but the background is overexposed. 

When I try to select background (or subject) mask, LR selects out most of the people except two. How do I add their faces to the groiup subject mask (or delete it from the background mask?

As well, when I select "Background" on the mask slider and make changes to the sky, when I return to the photo the group is now badly underexposed. I'm doing something worng but Lr is not separating the background from the people when I make these separatd adjustments. 

Any help, please?