Classical music is expected to instrumental. However, audiences generally expect some vocals as part of the performance in most other genres --- certainly rock and pop, and usually for folk music as well.

There *are* folk traditions, of course, which emphasize intrumental tunes. I think, however, that all-instrumental programs are less likely to be accepted in a "sit-down" concert setting than for, say, a well-informed group of folk dancing enthusiasts.

I'd advise Charley F & company to try singing; maybe a couple of group members will take to it more readily than others. You don't have to have a beautiful or a "trained" voice, as long as you convey your enthusiasm and your love of the music.