I ran a folk concert series for 27 years, and during that time, I also ran a summer folk festival. I never booked a completely instrumental band... not even for dances. I booked unaccompanied singers, but not unsinging bands. I don't think that my audience would have sat through a whole evening of instrumentals, no matter how good the band was.

That said, the singers in many of the bands weren't great singers. In some of them, I figured that they drew straws and the one who lost had to sing. But, that wasn't critical. Many string bands over here in the Yew Ess Aye weren't reno3wned for being great singers, but they brought energy, and a sense of humor to their music.

I'd encourage you to get whoever seems to be the best (and willing) singer in your group to sing leads, and any passable singers in the band to join in on choruses on songs that have them.

Jerry