Notes
Bar sugar (also called superfine) is a finer grade than granulated sugar which means that bar sugar dissolves much more quickly than granulated. You can use it in place of granulated sugar in recipes with no problems. You won't have sugar granules sitting undissolved at the bottom of the mixing cup.
Don't use powerdered or confectioner's sugar. This stuff has cornstarch added to prevent clumping which makes your drink taste like, well, cornstarch.
Bar sugar can be found at the grocery store. It goes by a variety of names, such as baker’s sugar, bar sugar, superfine sugar, berry sugar, and, in Britain, castor sugar. I usually refer to it as bar sugar.
I use C&H Baker's Sugar which works great. You don't want to use powdered or confectioner's sugar--this stuff has corn starch in it to prevent clumping. That would not do too well in a drink.
Also, Domino Superfine Sugar will work just fine.
Finally, C&H Superfine Sugar does very nicely. This one can be a bit harder to find.
Keep your bar sugar in a tightly sealed container, as it is prone to clumping. I keep mine in a glass canning jar.