it is not possible adding AD Users / Groups to the PRTG Administrators group. You will have to give the whole user group which was created based on the AD group administrative rights in PRTG.
Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.