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.
As the feature was implemented there was a discussion about the "order" of information showing up in the drop-down selector. MAC was identified as the most stable one to identify a client. That's why it is added instead of IP
But I agree it can be cumbersome to remember which client has which MAC once you filed the table (used the group management). One obvious solution is to write something in the comment section that helps you to remember which MAC is which client (can be a IP, hostname, anything).
I also think it would be nice to get some more information and like the idea of a tool tip (although this would not be usable on touchscreens): Pi-hole could display the information it already has about a client (shown in the dropdown; like MAC, IP, vendor) when hovering about a client entry in the table. Of course it is only usable for some clients, if you manually defined e.g. subnets or interfaces it won't give any additional information.