Speaking as a wanderer-in, if I find a session or folk club, singaround.. I try to go a couple of times in order to find the lie of the land, how is it organised?, take-your turn in a circle, put your name down, take a ticket, get pointed at, jump in..

Some are more organised some less so.
Some are acoustic music orientated, some mic'd or amp'd.
There is an irish/english music session near me that I frequent, but will not sing unless asked, that as payment for being able to listen to their skilled playing.
At other clubs, I do not tend to join in with others songs, unless the majority join in the chorius first, this on two counts, first I have a loud voice, and like to know the whole chorous in order to do the singer justice. Sometimes I do join in with a singer, but only if I know they don't mind (so please tell me if you mind!!), or if it's someones first shaky inroad to singing and they look as though they need help, bit then only if I know the song ..and then I still let them lead the pace and fade my voice down soas not to swamp theirs.
I don't mind people joining in with my singing, so long as it's my timing and my words!!!
I have broken many unwritten rules too it seems.. 'that's so-and-sos song' 'don't sing it when they are there'..
I treat the evening as an opportunity, perhaps to sing, perhaps to listen, perhaps to collect a new song or too, or hear how one should be sung, drink beer and cider, eat crisps and not be put off by the dominoes team, the skysports channel, a noisy group of revellers.... a noisy smoke extraction fan..
Each venue has its merits and demerits, I went to one in a pub-room on the wrong night (it was melodeon/squeeze-box night), and I was the only non-playing listener.... eventually some one asked me if I played, 'No', I said, 'I sing'.. and they let me sing!!! very convivial night... different place different night, acoustic night.. there were five of us, and one was the landlord!
we still enjoyed ourselves,
Sometimes the 'circles are too big, too slow, too organised, not organised enough, too noisy.. so instead of singing, try to collect a song, or try to insert 120 verses of that child ballad if they piss you off...then just don't go again!!.
My pet hate is 'sing something cheerful', 'less verses''not as depressing'. True , my song list has many dire tales in it, and some not so...traditional stuff as she is.
I will listen to your songs and respect your rendition, please give me the same courtesy. I try to 'ruffle' my pages a few 'turns' before mine, and not take over someone's singing of a song, or try to do a harmony if that throws the original singer's ear.
Optimum number... 5-10 for personal enjoyment, 10-15 for communal, 15-20+ it's an event and background 'chatter/noise has to be controlled or it becomes mic-night and sell tickets.
I seemed to have waffled a bit..sorry!