New User Signup means you can make your clients to sign up the newsletter sending function. And each sending number and price should depend on you. This is just for the Site Edition. Every new signer has the same permission as you. If you have purchase the site edition (the resell edition), you can give exact credits to your each account which you have already sold to your customers. In each account, your customers have the credit amount number, the expire date, and all the account can be controlled by you.