How do I synchronize subscribers from wordpress users into e-Newsletter members?

I cannot seem to locate the ability to easily add members who are already WordPress users.

I have folks that are subscribing via S2 Member and even though e-Newsletter is set to auto-subscribe them as members, it isn't happening. So I have a bunch of users that I need to mark as members - trying to find out if there is a better way to add them in rather than by manually entering their name & E-mail information.

Thanks!