It's sometimes been said that the bees will discard dry sugar, so it is not
a useful method of winter or early spring feeding.
 
If you put sugar on you might see some white crumbs being discarded at the
entrance.  Looks like the bees are tossing the sugar you gave them.  But
chances are, if you taste the crumbs, they will not be sweet, and it is
actually the leftovers of starch that was mixed in with the sugar.
 
There is plenty of moisture within the hive being given off by the bees
respiration, to get into the dry sugar (enough to make it edible), if it is
at the top of the hive.  This puts the moisture given off by the cluster to
good use, while keeping them alive til they can access stored or new honey.
 
An easy way to feed the sugar is in a rim, about 1-2" deep, set on the hive
beneath the inner cover or lid.  A piece of newspaper goes over the top of
the brood chamber, under the rim.  Then a hole is ripped out of the paper,
2-3 inches in diameter, centered on the cluster.  Dry granulated (table)
sugar then gets dumped in all around the hole, atop the newspaper.  You can
get three to five lbs. sugar in there.  Cover them up, and that's about it.
Gradually the sugar becomes moistened on the surface and the bees will feed
on it well.  A few pounds will last them 3-4 weeks.  Eventually the sugar
clumps together; the newspaper is there initially to keep the granules from
falling down between the combs, and the bees will chew away the newspaper
as they feed.
 
You can also drill a couple of 5/8"-or-so holes in the front edge of the
rim, and this acts as an effective upper entrance and vent.  These rims can
even go on at the beginning of cold weather, to supply a good upper
entrance, whether or not you need to put sugar in them later.
 
I have had great success with this plan, and it can reliably save colonies
that would otherwise starve out.