[gnat]

There is often a leader, ranking highest in some combination of talent and age, agreed upon tacitly, who gets to tell people when to take a solo.[...] Such leaders can establish a feeling or a culture for the group.

Correct. It's not always a formally-assigned role. In experienced jam groups, the next soloist is decided with looks and is much more democratic. It's often not a fixed role; the person singing the song often takes that conductor role and nods or calls.

I'm an experienced jammer, but I regularly encounter folks who need to be shown the basics (lead vs backup, how you know when to take your turn, how a song is chosen, etc.). Surprisingly few classes teach this, preferring instead "this is a G chord, etc." Probably the best national resource is Pete Wernick method classes: https://www.drbanjo.com/new-jammers.php (I have no financial interest in saying this, just a selfish interest in having more sensitive jammers :)