I wanted to close this topic by summarizing my “Solution”
Keyboard buffer forklift of the invitation data into a Mac Numbers spreadsheet (it readily accepted and columnized the table data from the CMS list view of invites.) This goes page by page. I had about 15 of them.
I then copied the single column of email addresses into the MailChimp account and created a “Re-invite” Campaign with some other stuff, like how to ask for help, etc.
This circulated to something over 100 (de-duped) addresses.
Yup. Thanks! I was really hoping I was just missing something simple…
Anecdotally — it appears that single invitations are transmitted reliably. Using the multiple invite feature (addresses separated by commas) might be failing.
It’t not tested with the version of WP operating, but I installed it. Seems to have no issues. Love that it can retroactively join existing members as I go through and update group settings.