You could try contacting the National Infantry Museum. https://nationalinfantrymuseum.org/
Also social media Facebook has a project group worth looking at. https://www.facebook.com/themilitaryyearbookproject/photos
Dear Ms. Wills,
Thank you for posting your request on History Hub!
We searched the National Archives Catalog and located the series Yearbooks, 1966-1969 in the Records of the U.S. Continental Army Command (Record Group 546) that includes Fort Benning, Georgia. These records have not been digitized. For access to and information about these records, please contact the National Archives at College Park - Textual Reference (RDT2) at email@example.com.
Due to the COVID-19 pandemic and pursuant to guidance received from the Office of Management and Budget (OMB), NARA has adjusted its normal operations to balance the need of completing its mission-critical work while also adhering to the recommended social distancing for the safety of NARA staff. As a result of this re-prioritization of activities, you may experience a delay in receiving an initial acknowledgement as well as a substantive response to your reference request from RDT2. We apologize for this inconvenience and appreciate your understanding and patience.
Next, we searched WorldCat and located several records for Fort Benning AND yearbooks that may be of interest to you. We suggest that you contact your local public library for more information.
We hope this is helpful. Best of luck with your family research!