Marion, it's been my experience that a lot of people go off key from the first chord that you give them. Let's say the song is in "C". So you strum a "C" chord, hoping that they'll start singing in 'C", but I'll bet you that they're singing in "F". "C" is the lead-in chord for songs in the key of "F", just as "A" or "A7" is the lead-in chord for songs in the key of "D", or as "G" or "G7" is the lead-in for song in the key of "C". I don't know what the musical terminology for this is, since I can't read music. If you want them to sing in "C", strum a "G7" chord to start them off.
All the best.
Seamus