Oh, that functionality is on the very TOP of the page!  I didn't notice that -- I was only seeing the headers (filters -- plus the Send Mail functional button) above each column.  Thanks!

OK, new problem/question:  I selected all at the top of the column, when it appeared to only be choosing among the people who were not yet invited.  Unfortunately, it appears to have also selected those who have ALREADY RSVP'd! :-(  So, how do I see those people separately, and change their status back to that?

Click the Checkbox on the stage that you want to select, and it will only select the prospects in that stage.

Thanks, but that shows no one.  Yet there were previously over 20 confirmed attendees!  In short, it's changed EVERYBODY's status to stage #1 -- it's erased their history.  Is there any way to revert the Yes/Confirmed people (I don't know who all of them are)?

@Robert Padulo

We keep a history of each prospect in any pipeline. If you open a prospect and then click "Activity", you'll see a history of all events for that prospect. 

In this case you'll have to go 1 by 1 to check since they were bulk moved like this, but each of those Activity tabs will show the history of their emails and stage moves, so you should be able to ascertain from that.

Looking at your event, for example, if you open the activity tab for the "Robert Padulo" prospect, you will see what you are looking for:
a "Moved to stage
Event RSVP (“Angel & Venture Capital Investing”): RSVP - Yes" under the more recent "Moved to stage"

OK, Understood.  But I don't see "Activity" anywhere -- where on the page is it?

First you will need to click the prospect in the pipeline to bring up the slideover, then you will see the details for that prospect. There will be an activity tab there, see my screenshot.
In this activity tab, this will be what you are looking for in your case, a "Moved to stage.... RSVP - Yes" a few activities down

But where is THAT page?  That doesn't look like the Events page I'm on. 

Oh, I found it.

New Q:  Even though I can change each of the 20+ manually, THEY still received the invite a 2nd time and believe they must RSVP again.  If they do, and I've already manually changed their status as confirmed yes, will anything get screwed up?

I'm making you a video to show how to get around this. One moment.

On that Activity tab, if it says "Event RSVP..." -- does that mean that they in fact DID confirm Yes they will attend?

You can prevent the automatic email send by doing this:
https://www.loom.com/share/539acbc50409419fa7763d82870db8ca

Finally, is there a way to resend- the previously-confirmed a new confirmation, so they realize they do not have to re-RSVP again?

If you see something like this in the Activity tab, that means they RSVP'd.

re. "resend the previously confirmed a new confirmation": You can use the Event pipeline like any pipeline. It is easy to send out a bulk email to whomever you want.

To be clear about the automation sequence:  IF someone RSVP's yes to attend, they will NOT get subsquent emails encouraging them to RSVP -- right?  Only those who have not yet RSVP'd will get those followups?

If you move somebody out of a stage and then back into a stage with an automated email, they will get another email unless you disable the automation.

To clarify, they ALL were (unfortunately) brought back to stage 1 - Invited.  And I gather there's nothing I can do about that -- they all re-received that 1st email.  If they now RSVP, their status should change to "RSVP - Yes."  Therefore, that person will NOT get the emails 2 & 3 inviting them to RSVP -- correct?
Secondly, if I manually change someone's status, will that also stop the automatic sequence of subsequent emails?

Correct, once they move out of the Invited stage they will not get the next emails. That goes for automated movements and manual movements.

OK, good.

But just to double-check, the only way I can see who was previously a RSVP-YES is to manually, separately, open each record one at a time, and scroll down through their history -- correct?

Correct