THis statement from your documentation is the issue:
"Private ideas portal users
Users can only register for a private portal if they use an email address that belongs to one of the domains included in the Employee portal setting."
THere should be a concept of a private portal (our software is highly competitive and many ideas could be leveeraged against us, making keeping this private vital.) where we have employees (Product team, support, customer success) that need access and the ability to comment privately on ideas before they are shared with the entire portal audience. THe Portal audience would be customers who self register, and we would like that registration to be restricted by customer domain. Right now, if we use the domain restriction, those users are EMPLOYEES and we can use VIsible to employees to hide ideas that arent ready for voting or "public" consumption.
Our only recourse is to turn off registration and import the users directly which seems counter productive and heavy handed given the self serve nature of this portal.
If we could just have a field above USERS which are valid domains (same as the EMployees domain field) that self registering users domains are checked against, this would resolve our issue.