This is a bit of a kludge in SharePoint 2007.
In your site collection how do you move your search scope from one display group to another? Logically you would expect there to be an option in the context menu (i.e. Site Settings -> Search Scopes [Under Site Collection Administration]) of the search scope, or be able to change it in the subsequent page when clicking the search scope.
That isnt the way. Here is how you would have to do it.
1. On the Search Scopes Page click on the name of the display group that you want your search scope to be in.
2. You will be redirected to the "Edit Scope Display Group Page"
3. In the scopes section check the Search Scope you want to appear in that display group.
Side note: If you want your search scope to be the default in your site make sure you make it the Default Scope in the Edit Scope Dispaly Group Page. Additionally ensure that in Site Settings -> Search Settings 'Use custom scopes ...' is selected
Cross-posted from tariqayad.com