I've followed the instructions to setup a Grouped Product which has two Simple Products both in stock and quantity levels set. Neither have custom options. The grouped product shows the table of two product in the correct place but beneath that the "add-to-box" is empty. I have re-indexed the date and cleared cache.
The problem was with this extension I had installed http://www.magentocommerce.com/magento-connect/call-for-price-9.html
For some reason it had a conditional statement to remove the Add to Cart button if showing a Grouped Product.