As I understand it, it's impossible.  In other threads, I've seen the suggestion that the appropriate thing to do when another character wants to join is to give immediately, and then launch a follow up conflict.  The problem I have is that once a conflict resolves, the stakes are supposed to resolve.  The example that is troubling me is deadly conflicts, where if one side gives, there's no point to the follow up conflict.  I'm specifically thinking about the person who mentioned their dog being attacked by the daughter of a man they killed.  His friends were willing to stand aside and see if he could persuade her not to kill him, but due to the conflict rules, they felt that they had to jump in at the beginning, or they'd have to stand aside and watch their friend get killed.  Having him give and launching a follow-up conflict wouldn't be useful, because then he'd be dead.

As I write this, it occurs to me that it might be solvable by being more careful with the stakes you choose.  If the stakes had been chosen as 'Do I persuade her not to kill me'  instead of 'Does she kill me'  then giving would have been possible.  Is this the right approach?  If so, it might be worth putting in a note about it when you discuss how to set stakes.  Is there another way allow characters to join a conflict they were sitting out of?

I think you're exactly right. Vincent spilled some electronic ink, here, about pushing for smaller stakes in conflicts.  It has made it into the new printing, so it sounds like the advice you wanted will be there for everyone now.

As far as I know, there's no official way to join a conflict in progress.

Hope that helps.

Here's another piece of new text, maybe I already posted it here but maybe I didn't:

A follow-up conflict is simply a new conflict that follows on the one just ended. In general you treat it exactly as you would any other, but it does have a few special considerations:
- It counts as a follow-up conflict only if its stakes follow directly from the previous conflict’s resolution.
- Its stakes can be the same as the previous conflict’s stakes only if all three of its participants, its stage as set, and its opening arena are different. That is, if your character tries to talk my character into admitting her sin, but fails, you can’t just try again. That conflict’s done. What you have to do if you want a follow-up with the same stakes is come back another time or catch her at some other place, with your friends to back you up - and this time it can’t be just talking.
- If you cut your losses in the previous conflict, Giving instead when it was your turn to Raise, you get to keep your single best die from that conflict. After you roll your dice for this conflict, add your reserved die (without rerolling it) to the mix.
- As the GM, I get an extra option, and it’s a good one. If nobody cares about my NPCs’ Fallout, when I roll my Fallout Dice, instead of calculating and choosing Fallout I simply give you the two highest dice to add into your side of the new conflict. You don’t reroll them, just put them straight in with your own dice. They’re the advantage you carry into the follow-up.

What I like to do when someone would interrupt a conflict is play time games. "Okay, everybody, this present conflict needs to resolve in the instant between when Br. Zeb notices what's happening and when he gets his gun out of its holster. Raise and see accordingly!"

Or alternately, "Br. Zeb interrupts the conflict and we all have to stand down, but it's not resolved. Let's pick it up again behind the stable the next afternoon, free from Br. Zeb's meddling..."

-Vincent