[SDPhantom]

You need to set the frame's event handler.

Example:

GuildLeader:SetScript("OnEvent",function(self,event)
    if event=="WHO_LIST_UPDATE" then
        for i=1,C_FriendList.GetNumWhoResults() do
            local info=C_FriendList.GetWhoInfo(i);
--          Do something with character data in table "info"
        end
 
--      Be nice and re-register the event so "/who" still works
        FriendsFrame:RegisterEvent("WHO_LIST_UPDATE")
    end
end);

See C_FriendList.GetWhoInfo() for the structure of the table returned containing character data.



As a side note, there are a couple misconceptions.
C_FriendList.SetWhoToUI() expects a boolean value, you're giving it a number.
GuildLeader:RegisterEvent() only takes one argument, the event you're registering.