According to the local newspaper, which did interviews with participants, the singers are affiliated with a group called "Public Acts of Christianity" (or something like that)
That's called Random Act of Culture. They recruit people from across the country via their website and if the plan looks good to them, they'll donate to their cause. Nothing to do with preaching or anything, just spreading the Christmas Cheer which is becoming, quite sadly, a lost art these days.
http://www.knightarts.org/random-acts-of-culture
Yiz