Webform send multiple email
You can send custom emails to addresses submitted in the form. When creating the form, you have to select 'email' for the field type. Once you've done that, go to the 'Emails' tab in the webform, and add a new email using the 'Component value' menu; you should see your Email field in there (you can add as many of these as you want). This sets it up to send an email to the email address entered in the form, in your case the friends' email addresses.

From there you can customise the email options.

Edit: Whoops, this answer is for Drupal 7 specifically, but (if I remember correctly) the option exists in Drupal 6 as well.