Should there be a semicolon after the third name and title?

I support this use of semicolons internally within the list, but think you do not need one after the third name, i.e., separating the list from the rest of the sentence. There, I think, a normal comma would suffice with no risk of leaving the reader confused, especially because the list items are so consistent and parallel.

So its punctuation is OK as drafted.

Suppose, for a moment, that instead of saying "Join X, Y, and Z, as they discuss bar-bar-bar," the sentence said this:

X, Y, and Z discuss bar-bar-bar.

In that case, there would clearly be no need for a comma after Z; indeed, putting a comma after Z would constitute punctuation overkill. Now suppose that we filled out the replacement sentence with additional detail, as follows:

X, job title A; Y, job title B; and Z, job title C discuss bar-bar-bar.

It seems to me that the only issue that arises under these circumstances is whether to put a comma after "job title C" or whether to leave it open (that is, completely unpunctuated). The argument for including a comma is that the job titles for X, Y, and Z function as appositives, and therefore setting them off with commas is an appropriate way to indicate the syntactical equality between each title and each corresponding personal name (X, Y, or Z). That is surely what we would have done if the text had read simply "X, job title A, discusses bar-bar-bar"—and the only thing that is different in our situation from that simpler hypothetical situation is that the we have a series of three discussers to account for instead of just one.

To clarify the pairing of persons to job titles, we introduced semicolons after the first two pairings. But there is no need to do the same thing after the third pairing—just as there is no reason to add a comma after Z in the simpler example "X, Y, and Z discuss bar-bar-bar." The comma after "job title C" is there because of the appositive structure of the sentence, not because its being the last of three items in a list requires such punctuation; there is no such requirement. So there should not be a semicolon after "job title C." On the other hand, in my opinion, there should be a comma "job title C" to mark the appositive status of "job title C."

Now let's return to the original wording that the poster asks about:

Join John Smith, President and CEO, X Corporation; Mark Jones, CFO, Y Corporation; and Doug James, CMO, Z Corporation, as they discuss changes in the corporate world after the pandemic.

The comma that completes the "CMO, Z Corporation" job title appositive in equivalence with the name "Doug James" should still be there for the reason given above. It doesn't matter that there is a further (arguable but less compelling) reason to have a comma after "Z Corporation" to mark the dividing line between the "Join X, Y, and Z" invitation and the "as they discuss ..." round-table description. We already have sufficient reason to put a comma there—and no reason to put a semicolon there.

One other possible change that might make the sentence read more clearly would be to replace the comma between the job title and the corporation name for each discusser with the word of. That is, you could lighten the punctuation load in the sentence by expressing it this way:

Join John Smith, President and CEO of X Corporation; Mark Jones, CFO of Y Corporation; and Doug James, CMO of Z Corporation, as they discuss changes in the corporate world after the pandemic.

This isn't a critical change, but I think that it does help the sentence flow a bit less choppily.

The names, titles and corporations are poorly expressed, and this has led to the question.

For clarity

Join John Smith, President and CEO [no comma] of X Corporation; Mark Jones, CFO [no comma] of Y Corporation, [required Oxford comma] and Doug James, CMO [no comma] of Z Corporation, as they discuss changes in the post-pandemic corporate world.

The list semicolon is used where the items within the list contain punctuation. The list semicolon is not used before a conjunction as either (i) the conjunction itself provides the separation or (ii) an Oxford comma is used.