Bert, with all due respect, I think it would be cumbersome in most song circles and sing-alongs for people to ask before every song, "Is harmony OK?"   If it's a setting where it's not "understood" whether you're supposed to sing along or not, of course it's good to ask if you may. But where the normal mode is for people to join in or if the singer has invited people to sing along, it makes more sense to me that if they don't want harmonies (or other back-up types of vocals), they should specify that.

:)

Genie