The Grounds were a handsome group. A lot refers to a group of people or things.

I totally agree with Richard. I've never heard this besides maybe a period drama set somewhere in England or in a novel. It sounds British to me. It would be used to describe a group of men but also women. It depends on the era in which it was set. "Those gentlemen consider themselves a handsome lot, I disagree." "That young lady belonged to a handsome lot but they were too bashful."

Meh... but keep in mind that this is literary writing. I wouldn't use either "a handsome lot" or even "a handsome group" in ordinary speech or even ordinary writing. It's still colloquial to describe a single male as "handsome" but to describe a group of people as "handsome" is much less common.

Are The Grounds a group of people, or a family? I think it could be written again like this: "The Grounds were all handsome, blue-eyed, fair-haired, clever and ambitious" or " All The Grounds were handsome, blue-eyed, fair-haired, clever and ambitious"