CiviRules prevents adding new contact
Martin, for the send email action you need the Email API extension: https://civicrm.org/extensions/e-mail-api.
Once you install that, the send email action should be available in CiviRules.

I have just added a rule to my local installation as shown in the screen print, and it works in my installation (CiviCRM 4.7.25, CiviRules 1.13). 


I have created a group Web Contacts, and when I add a contact to that group the email is send out.

So it is kind of hard to see what is wrong on your installation? Perhaps a first step is to configure the rule as I have?