OnePlus recently ended invites for their OnePlus One to celebrate the phones announcement one year ago. In that blog post they also mention that when the OnePlus 2 is released later this year you'll need an invite to get one:
"Yes, the 2 will initially launch with invites. We’re committed to maintaining razor-thin margins in order to give as much value as possible back to our users, and this drastically increases our risk. The OnePlus 2 will bring the challenges that come along with a brand new product, and initially, our invite system will help us to manage that risk. It also helps us to be sure that every OnePlus 2 user gets the amazing experience that they deserve."
"Yes, the 2 will initially launch with invites. We’re committed to maintaining razor-thin margins in order to give as much value as possible back to our users, and this drastically increases our risk. The OnePlus 2 will bring the challenges that come along with a brand new product, and initially, our invite system will help us to manage that risk. It also helps us to be sure that every OnePlus 2 user gets the amazing experience that they deserve."