Like any cults, yes you do need an invite but they practically let anyone in really.
if you dont remember how to join a cult. Go to the My NS menu up top, hit cults and then click view all official cults. Sticker Cult should be up in the top few.
but for your convience, heres the link to the one and only sticker cult. http://www.newschoolers.com/web/content/viewcult/action/view/id/3745/