Most of the sessions I attend are Bluegrass-oriented, where the emphasis is more on picking and strumming than singing. Partly as a result, you have a wide variety ranging from outstanding singers to those who can barely carry a tune. Often the tempo, chord progressions, and verse changes in the song are tied closely to the vocal. I encourage all singers to sing with gusto, even the bad ones, partly because the music is tied to the vocal and needs to be heard, but also because even bad singers like to sing. My motto regarding singing is "if you are going to fail, fail boldly". What's surprising to me is, when some bad singers are encouraged to do it with gusto, their confidence increases and they improve tremendously. Others never will. But any ongoing jam or song circle is a little community, and tolerating shortcomings is partly an act of kindness that tends to bond the group. Now, if we were strictly vocalizing, and if harmonies were jarred apart by someone off key, I might feel differently.
Regarding notes, songs on paper, yes I do that. It is one way to try out new material, play a song with lots of verses (even my own songs), and bring some variety to a jam. If you rely on memory only, and you attend a weekly jam, the song selection can get pretty repetitious pretty fast. And what's a jam if not a place to try things out? It's not a performance where professionalism and your reputation are at stake.

LEJ