archivearchie on "Problem adding new User"

I am not sure this is the sort of question I should put to the forum, but can find no way of contacting wordpress.com directly.
I have tried adding a new User to my blog, but am told they are not a wordpress user. I know this is not the case as he has left comments on the blog and indeed has set up his own wordpress.com blog. The only explanation I have thought of is that he has changed his email address recently. However, neither the old or new address work.