Measuring with a hydrometer will always work. It is possible that the pulp will affect the reading, but it should be very minor. As long as there isn't a lot of pulp floating on top, you shouldn't have any problems (if it is, just scoop it off). If the pulp is suspended in the cider, that means its density is the same as the juice and shouldn't really affect the reading.
You don't have to add any, cider is usually as simple as just juice and yeast. You can add sugar if you like, but some would call that an apple wine, not a cider. (others might say it depends on how much sugar you add)
Check out the recipe page if you want something more then just plain dry cider:
Cider Recipes - HomeBrewTalk.com
Otherwise, just pitch the yeast and wait.
Adding sugar will increase your alcohol content, so I'd say figure out how strong you want it. After it is done fermenting it, you can also look into adding sugar to carbonate, or stablize and backsweeten.
What kind of drink are you looking for?
How strong? How Sweet? Do you want it carbonated? Do you want some other flavors other then apple?