I sing harmonies on most of our set, We do 'When You Walk In The Room, and I sing the same lyrics as the lead singer just a third above. This is great for him as he always forgets the words.
This is not always a good thing though, as I sing lead on a few numbers too, and last week I sang 'Brown Eyed Girl' the whole song a third above! Try as I might, I couldnt get my voice back on key! force of habit!!