Sheet music is not necessary--or even desirable for most folk music. Many instrumentalists only need be told the key --in country, bluegrass etc.--and they can accompany you--and even enjoy doing so--and like taking breaks.

I certainly also second the motion on joining a choral group--though that isn't the thrust of this thread. I've sung in madrigal groups, sea chantey groups, men's choruses, mixed groups, big groups, small groups, 1 on a part, 12 in the group, 40 in the group, 180 in the group etc.--and it's all endlessly satisfying.

If you want to be accepted at bluegrass sessions however, it's fairly important to have some vaguely bluegrass-type instrument you can play a bit---just to be able to get into the inner circle, where the music is happening. Just holding an instrument--and knowing what key you sing a given song in--is pretty important. For some reason, they don't seem to take seriously anybody who does not hold an instrument. At least that's been my experience. Obviously, also the more lyrics you know, the more demand you'll be in.

But if you're throwing a music party, you can certainly ask the instrumentalists to accompany you--just tell them the key for whatever song you're doing.

And as you know, the ability to sing harmony is also a big plus. I know you do this--I was really impressed by your singing at the Getaway.