You'll need someone familiar with modifying frontend templates in Magento such as a frontend engineer to help you with doing this, however it is relatively easy.
To limit store credit usage by customer group simply copy /app/design/frontend/base/default/template/customerbalance/checkout/onepage/payment/additional.phtml to your theme folder in the same structure and wrap the entire file contents with with:
<?php if (Mage::getSingleton('customer/session')->isLoggedIn() && Mage::getSingleton('customer/session')->getCustomerGroupId() == 1234): ?>
<?php endif; ?>
and replace the 1234 with your customer group.