Interesting, this just about works, I have to move the 3rd column here back into the index:
julia> groupreduce(max, testtbl, :a; select=(3,:b))
1-d NDSparse with 3 values (2-tuples):
a │ 2 3
──┼────────────
1 │ 0.630628 3
2 │ 0.521345 4
3 │ 0.816977 8