Be prepared to issue receipts for people who need them. Give poeple an excuse to get up, stretch their legs. Already Hilarious Word Jar Readings for example. People told to line up in a row. Offer one set for this.
Some people attend church just to sing.
Some people WILL come to the coffeehouse just because they know they ll be able to sing folk songs and new songs in a group. Make it clear it s only for this set. Some performers can t perform if people start singing with them. Once sing-along starts in an audience, and clapping to the music, it is difficult to get audience to stop that. Make the evening escalate with excitement.