How do you add thank you pages to the landing pg form test group?
Elizabeth, are the "thank you" pages in question Marketo landing pages? If so, make sure they are approved and under the same parent program. You then right click the program and select "new local asset" > "Test Group" and select the landing pages you wish to test.
This works the same way you add any page.
But your LP Group should be just for landing pages. Don't add these to actual landing pages.