well, you can't group by top and then by bottom...they are just entries, I would think that you would group by a formula. If the entries are just top and bottom, then you could create a formula like:
left({table.field}, 3)
the display for the group would be something like:
if left(table.field},3) = "top" then
"Top"
else
"Bottom"
if the entries are more varied, obviously the formula will get more complex, but you would need to find a way to split entries correctly into different groups.
HTH