Invites are going to go out to certain forum members based on how long they have had an account for. The older your account, the higher your ranking. You also must have a minimum amount of posts. OnePlus says that the required number of posts is "less than you think," so perhaps having written a post or two will be enough for you to be considered. However, some of these posts need to be recent. OnePlus wants to be around for a long time, so they need more than just fair-weathered fans.
OnePlus is forcing those who get an invite to use it within 24 hours, or else it will go to the next person in the rankings. This is smart because it prevents a secondary market in invites from starting.
The company is also working on a pair of contests involving the 64GB model, and that is sure to draw a lot of attention. The reason for the plethora of contents has to do with saving money. CEO Peter Lau said last week that the OnePlus One is being sold at cost. To make that work out, the company eschews national advertising campaigns, deciding to go with community partners and online advertising.
source: OnePlus via AndroidCentral