Add the yeast nutrient, you'll get a much healthier fermentation with it.
Nutrient is usually added at the beginning of primary fermentation, but there is also staggered nutrient additions (SNA). SNA is adding part of the nutrient at various stages of fermentation instead of adding it all in 1 dose at the beginning. For cider, either will work. You can learn more about SNA here:
If you're adding to primary prior to yeast pitch, I'd stir the hell out of it to both mix well & aerate the must. If the fermentation has been going for a while, I'd mix the nutrient with a little boiled & cooled water & GENTLY stir it in, or rack onto it. Adding a dry powder to an active fermentation will provide nucleation points & cause the CO2 to suddenly come out of solution...
Meaning it can cause quite an eruption of must to come shooting out the top of the carbouy, much like a wet, sticky, boozy volcano.
Hope this info helps.