You usually invert sugar to increase it's apparent sweetness. IE: Break the disaccharide sucrose into the monosaccharides glucose and fructose. The yeast may be able to consume the monosaccharides a little faster then the disaccharide, but not by much. I don't see any actual point in inverting, as the sugar is already in a form that the yeast can eat.
If you meant dissolve and not invert, then you shouldn't need to. That is, unless you want to add a very large amount of sugar. Up to about 3 lb's you should just be able to shake or stir and have the sugar completely dissolve. Over that, I'd gently heat the sugar in a pan with a gallon or so of cider so it is fully dissolved. Then mix that into the rest of the liquid.