manually add members to the Membership Plugin

How do I manually add members to the Membership Plugin.

How this site works is that Members apply for membership, are invoiced and them pay directly - not through an online payment gateway. I then need to add them manually through the back office. We are also moving this from a print publication online so I also need to load existing members.
I also therefore have set up a Form for people to apply to become members so don't need to use the membership Sign up page - how do I get rid of this?
How do I do this all?
Thanks

Roger