It's all a matter for the performer - they've got to make the audience feel comfortable about joining in - and be careful, as this simple act can be overdone if one treats the audience like children and therefore have the opposite of the desired efffect.
It's a knack like anything else we do, but, I put it down (loosely ) to coaxing intelligent people but remember to speak the bits beforehand which you want them to join in and give them some friendly patter, - don't assume just because everyone knew the chorus in a club last night that the club tonight automatically knows it.
So appeal to them gently " For those who don't know the chorus here's how it goes " and those who DO already know it can feel ..............hmm .......what's the word........will "content" do ?

All the best