Sending invitation to register as new user

Hello,
My website uses the ‘Commons in a Box’ theme with WordPress for its homepage. Sites I connected with it have different themes. How can I organize the following:

  1. I want to send out email invitations to select persons to register as new users with the website
  2. How do I manage the settings so that only invited people may register?

I have more questions but this here is it for a start :wink:
Claudia