Unable to invite Zoho user to workspace

I am unable to invite a Zoho user (pcedmunds) to my workspace. I have successfully added another Zoho user (markeford) but when I attempt to invite pcedmunds it says: Unable to invite user to the workspace. Paul appears to be in the system successfully (from my contacts screen shot). Why am I not able to invite him?

Please see the attached screen shots for more information.

Thank you for your help!