To arrange the order of Simple Products within a Grouped Product follow these steps
1) Log into your Magento Admin Panel
2) Navigate to CATALOG > MANAGE PRODUCTS
3) Locate the GROUPED product you need to sort
4) on the left side tabs within the product select "associated Products"
5) this will show a table of the simple products associated to it - the last column of which is "Position"
6) To order this change the numbers starting from "1" (will appear first) and moving up