I have sung in choirs and have never had a singing lesson in my life.

Admittedly both of the choirs I've sung in were non-auditioning, but I was the sop soloist in one of them and so can't have been that bad. I was invited a couple of years ago to audition for one of the more prestigious choirs in my area, but I didn't audition because I knew that I couldn't commit the time that would be required for all the concerts.
I know of some people who have successfully auditioned for the above choir having never had any formal training. I also know of some grade 8+ singers who have been turned away from the above choir. One of the things that is important in choirs is the ability to blend, and it seems that some of those who have been more heavily trained so that their style is more akin to that of an operatic soloist struggle to blend with other singers.
The choir I was in most recently did include some people who either could not read music or were poor sight readers, and they had to learn very quickly how to pick things up by ear because we would typically do 3 or 4 new items each week in addition to keeping other music ticking over. Those who couldn't keep up, or were uncomfortable with the religious content of the music, generally dropped out. In a choir which spends longer on individual pieces of music, poor sight reading skills of a lack of familiarity with musical notation would be less of an issue.
Some choir directors will cover very basic technical issues in rehersals (i.e. open throat, singing from above the note for top G/A/B etc) - but this is nowhere near what would get covered in singing lessons.