A little trick I use with "sit-in's" is to relate their voice to mine. I know what key I sing the songs in. After I hear a person do one song, I make a mental note of the relationship of that key to the one I would sing it in. Then on subsequent songs I can adjust up or down from "my" key, as the situation demands. This is not perfect but might be helpful when dealing with amateur singers. The pros will tell you what key they sing the song in.
In my old age I have become less and less tolerant of sit-ins. Just Saturday night I had a guy ask if he could sing a song with me (that's usually a red flag, because a pro will never ask, he will wait to be asked). It was slow at the time, so I said o.k. He told me a song. I asked what key.
He said it didn't matter. I knew I was in trouble.
I got of the subject a little didn't I?
DonM