Will look at this again in the morning - tried a version not requiring a log in, and did get the comma separated list - 7 and 3.
The problem I have is that the validation isn't preventing a new user being added if they are using the same email address or username already used by someone else.
What it should be doing is checking it against the database table, and if the email address or user name typed in the form is already used in the database table, displaying error messages to that effect, as it does on the link above, with the email address or username I posted.
I have attached a copy of a page that does work.