Thankfully the index is organized not only by battles, places and names of even obscure individuals but also by units. The Eighteenth Georgia Battalion (as opposed to the Eighteenth Georgia Regiment) is mentioned on page 557 as having constituted part of the garrison of Battery Wagner (Morris Island, South Carolina) during the fifty-five day siege of Battery Wagner. The service there was three days rotation. They were present on July 11-13 and July 15-16.

If your relative was in the Eighteenth Georgia Regiment, they're not mentioned in the book.