I can't think of any other way off hand. I'm assuming you don't like cells because of edge cases where two people could be near each other but in different cell

I can't think of any other way off hand. I'm assuming you don't like cells because of edge cases where two people could be near each other but in different cells? If the cells were carefully selected it might not be too big of a problem. For example, you could put each college campus in its own cell. But that could take a while and you'd have to drop the one-mile guarantee. I'll let you know if I think of anything else. I don't think it has to be a dealbreaker for an otherwise great idea!