Pre-registration in web apps

Suppose a member management app where the user enters info about members. It's possible for such members to be added without an email address.

Now if a member wants to create an account on this app what it is the best way to associate previous member info with this newly registered user.

Example:

A manager registers "John Doe" as a member. Sometime later John decides to jıin this site and registers by creating an account. So how can I make sure this is the same John so I can give access to him.

Any ideas?