OK, when Team Plasma gave their first speech out in Acumula Town in the game, was N already with them, or after Team Plasma gave their speech, he agreed with them and then took there side?
He was already part of Team Plasma because if you watch the intro of B/W you can see Mr.N's story.
For more info go here-- http://www.serebii.net/blackwhite/plasma.shtml
Hope this helps =P