Ah yep, NetWorkingDays() is for calculating the working days between two dates.
To add X business days to a date, use Workday(StartDate, NumberOfDaysToAdd)
(optionally you can add a third parameter for a list of holidays - I usually keep this list in a table in my doc, so I can reference it in multiple business-day-related formulas. If you do it like this, your third parameter would be HolidaysTable.HolidayDate if you call the date column in the holidays table HolidayDate. For bonus points, I have a doc in my workspace that has a table of all the holidays observed by my organization, and then use Cross-Doc to pipe it into any docs that need business day math, so that I can update the holiday list in one place and have it automatically propagate everywhere else)