Our singing tradition is, with very few exceptions, a solo one (wassails, sea shanties and such like excluded).
Time and again I have seen peoples' efforts naused up by insensitive egotists who want to show that they know the songs as well as the singer.
Why should singers have to ask that people should not join in - it should go without saying and it is arrogant to assume otherwise.
Please tell me what Walter Pardon should have done when people joined in with his singing (he was part of a sing-around in his market town of North Walsham for years); should the onus have been on him stop people making a free-for-all of his songs?
The best example of an audience on an ego-trip was one night at a London club - not that long ago, when the singer embarked on Scarborough Fair. He had chosen (in his stupidity I suppose) to use the beautiful minorised Kidson tune - the audience couldn't make up their mind - half of them joined in (verse and chorus) with the well-known MacColl/Littlewood version collected from Mark Anderson while the other half launched into the Simon and Garfunkel adaptation - result, an absolute cacophony   
It is not even a question of joining in on the songs, a certain amount of sensitivity is required on choruses.
Walter got extremely upset when some audiences decided to both harmonise and slow down choruses making them clever-clever dirges.
Unless you wish to reduce folk-singing to community singing before kick-off the individual singer - not the audience, must call the shots - he/she has to be the interpreter - the audience is there to listen and to help out if invited.
I think I'm beginning to see why the club scene is in such a mess if this is representative of current thinking.
Jim Carroll