<cfscript>

foo = queryNew(
    "id,name,group",
    "Integer,Varchar,varchar",
    [
        [1,"One", "A"],
        [2,"Two", "A"],
        [3,"Three", "B"],
        [4,"Four", "C"],
        [5,"Five", "C"],
        [5,"Six", "C"]
    ]
);


choo = foo.reduce(function(previous, row) {
    previous[row.group] = previous.keyExists(row.group) ? previous[row.group]+1 : 1;
    return previous;
}, {});

writeDump(choo);
</cfscript>