Should be easy, yes, but can be difficult. When you notice no email is sent, but no error message is delivered. Maybe this can help the ones who are having simular problems:
There are 2 settings you have to make in your Sharepoint user group, the first one is to give everyone view acces to this group. Like this:
Then the user group should have read permissions on the site.
Now the workflow can send email's to sharepoint groups.
Mark Deraeve