user subscriptions

I don’t know what Version I’m on and don’t know how to find out.

I have set up a site and am trying to add users as subscribers and for most (not all) I get the error message “Usder Not found! Users need to register here.”

What does this mean? Register where@ HOw? Why?

I have searched FAQs and documentation etc in vain for an explanation of the different types of user and why I have this problem – please advise.

Thanks.