Google-groups – User trying to join Google Groups is rejected as an existing member when not on the list

I'm an admin for Google Groups list and I'm having a problem where a user is not being allowed to join, with the rejection stating that the person is already a member.

When I look at the members list, she is not there. But she is on the list that have received invitations. I removed her from that list and asked her to resend her subscription request to the list. She then got a rejection email along the lines of:

Delivery to the following recipient failed permanently:

example-news+subscribe@googlegroups.com

Technical details of permanent failure:
Failed to create an account for user@example.com – EMAIL_EXISTS

I've detailed the question in Google's forum with full header, but the gist has been quoted here.

Are there any other steps that I can/should take? I've searched in the Google Groups forums for anything that could help and came up empty.

Or will my issue need the attention of and escalation to a Google employee?