You might get a better response on the ML than here, but...

You could mod this for your needs. It expects the rows to be
sub-blocks, and it doesn't remove the column you're grouping by, but
it's a general solution. I think this was something I did early in my
REBOLing, so it could probably be greatly improved as well.

    group-by: func [  ; See SPLIT
        {Returns a block of blocks+sub-blocks with items partitioned by
        matching index elements in each sub-block.}
        block  [any-block!] "A block of series values"
        index  [integer!] "Index of sub-series value to compare, for grouping."
        /part "Compare part of the series, rather than a single value."
            range [number!]
        /local result val
    ][
        result: copy []
        either part [
            ; First, build up a list of keys, with empty blocks to go with each.
            foreach item block [
                val: copy/part at item index range
                if not find/only/skip result :val 2 [repend result [:val copy []]]
            ]
            ; Find the correct place (key) to put an item, and add it there.
            foreach item block [
                val: copy/part at item index range
                append/only select/only result :val item
            ]
        ][
            ; First, build up a list of keys, with empty blocks to go with each.
            foreach item block [
                val: item/:index
                if not find/skip result :val 2 [repend result [:val copy []]]
            ]
            ; Find the correct place (key) to put an item, and add it there.
            foreach item block [
                val: item/:index
                append/only select result val item
            ]
        ]
        result
    ]

    group-by [[a 1 2] [b 2 3] [a 2 4] [c 2 3] [b 1 5]] 1
    group-by [[a 1 2] [b 2 3] [a 2 4] [c 2 3] [b 1 5] [c 2 4]] 2
    group-by [[a 1 2] [b 2 3] [a 2 4] [c 2 3] [b 1 5] [c 2 4]] 3
    group-by/part [[a 1 2] [b 2 3] [a 2 4] [c 2 3] [b 1 5] [c 2 4]] 1 2
    group-by/part ["ABC" "AAC" "ABD"] 1 2

--Gregg