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Do all invitees have this problem, or just a few?

I think you may need more detail from your invitees. Specifically the exact text of the error message being reported by their email service. But here are some other ways to check it out:

One possibility is that they are posting to an address that isn't even Groups.io. A version of this is that they're getting that much right but have a typo or other problem in the group name. To diagnose this, ask them to post again but CC you so that you can inspect the To address in their message.

Another possibility is that they didn't actually accept the invitation, and they're getting back a "not a member" error from Groups.io. In this case the Invitation page should show them as "sent" but not "accepted". Or they did accept the invitation, but are sending from a different address. This case you can diagnose by looking at your group's Activity logs. Attempted postings by non-members should be logged there. This is assuming you have not elected to allow non-subscribers to post.

Shal