Hi Susanne.
First you need to identify your form user and what group they belong to. You can do this via SharePoint 2007 or I mostly use an Active Directory Web Service.
Then you can either set your rules per field, or in InfoPath 2007, you can set a whole view read-only. You can make copies of your views and then route people to the appropriate view based on who they are and their group. Or you could use one view and have sections with conditional formatting.