Marion,

On the rare occasions I've been in a church for something other than concerts or a song circle, the pianist or organist plays the last line of a hymn as the intro. It seems to give people a chance to figure out where to sing. I carried that over into Xmas carols whether playing piano (my apologies & condolences to those who've heard) or guitar. It's been very effective for me in unrehearsed situations.

If the lead singer (e.g., someone with a microphone) can't find the right place to sing, I'd suffer the occasion, feign desperate illness if there's more than one song, and never, ever work with that person again until I can find and play anything in any key.

I have met people who sing in one key and one key only. And if you don't know what it is or can't find it, accompanying them is a lost cause. Other people change key when they sing louder, to a higher key as the throat is tightened to give more force to the air.

IMHO, if you value your musical reputation, steer clear of these problems until you can deal with them one way or another. Just bow out with profuse thanks for the honour of being asked, gallons of grace, then run, don't walk, fast and far! Assertiveness courses are a wonderful resource: no rehearsal, no Marion.

~2 cents for free~
CH