Email Notification Preferences

You may not be aware but even for external email addresses you can list the recipients by field name in square brackets. For example.

[email 1]
[email 2]
[email 3]

So you can have a table of recipients and roll them all up in a Combined text summary field.  The make 150!  fields to parse out the separate email addresses.   Yes, A bit of a grind to do tat, but it can be one in an hour as its very repetitive to make the formula fields.

The limit for a combined text summary field is 150 records, so you are OK up to 150 names without needing to resort to a separate table.

So that reduces the issue into how to have these non-users self maintain that table.  It could be done with a Pipeline which is accessing a mailbox and looking say for a key word in the subject line like

Add
Remove.

and the Pipeline would either add a record or delete a record.

So I do think that it is possible but you might need some help setting up your first pipeline.  But its a really good use case which is safe to test on and learn Pipelines.