Any performer worth their salt has learned to "read" an audience. As a very wise lady told me once......If they want to join in....let them! It's a compliment to your engagement with the audience. If they don't want to join in....don't force them and go all happy clappy. If you tell them not to join in you risk alienating your following...however many there be! And they must be listening to you for a reason?
Are your audience listening to you in respectful silence, or have they gone to sleep?
The same goes for an audience.......you must be listening for a reason, or joining in for a reason?