Well, there's ballads, and then there's ballads. Are you looking for trad ballads? Recently composed? Do you lean towards Irish? (Or English, Scots or American?)

That said, I've had a lot of luck getting audiences to join in on Golden Vanity (Lowlands), Cruel Sister, Hungry Child, Prickle-Holley Bush, Pleasant and Delightful . . .

There's a lot of stuff out there with choruses or repeated last lines. A place to start browsing might be Rise Up Singing. Just don't take what's written there as gospel -- I hate how sing-arounds can devolve into people singing songs out of that book that they don't know or half-remember.