Under Group Managment you can add clients from the dropdown list for allocation to groups.
However, even though the text on the page recommends IP over MAC, when entries are selected and added from the list the MAC is used and no further detail is supplied in the comment box nor when hovering over the entry in the clients list.
This makes using the drop-down to add clients hard work as you can only see additional client info at the very beginning and later have to look up the MAC to confirm which client is which.
I am requesting that clients be added by IP address as the simplest solution. Other possible, but more involved, solutions would be to add details in the comment field or supply a choice of which value is to be used to add the client.
This seems to happen when the pihole is not the DHCP server but I have not been able to confirm this as I would have to change my dhcp design.