How would you advertise karaoke?
If you’re doing posters and/or flyers you could say that there’s a prize for the first person to be brave and get up there/prize for the table with the most willing singers. I think incentives are key.
One of the best advertising tricks in the trade is to put your posters up in the toilets of local pubs, on the inside of the cubicle door. It’s very hard NOT to read them fully, then.
You could also capitalise on the rash of talent shows out at the moment, for example, although you can’t use trademarked names you could print “Think you can do better than *insert popular tv contestant’s name here*?