Dynamic endpoints with comma separated email addresses are the preferred method for addressing notification recipients. They allow all recipients to see which other recipients have received the notification.
The downside is any syntax error (e.g. typing a semi colon rather than a comma) invalidates the whole list, meaning that any updates to the recipient list are risky and need to be tested.
Some basic level of validation and formatting in the attribute data reference would be very handy.
I would imagine it might work in some ways like the URI builder dialog and in some ways like the array type value dialog.