We've heard a fair bit about the invite system that will be used for the initial OnePlus One purchases, and it seems the company has been offering some updates on how the process will work. For one, OnePlus is listening to feedback and they are making every effort to offer them without worrying about people re-selling them to others.
OnePlus didn't go into all the specifics about how they plan to keep people from selling the invitations, however they did mention the "re-sale of invites will not be a major issue." The few bits they discussed here include how the system was designed to counter the potential for resale, and they will have "full control over distribution and expiry times."
In terms of the expirations -- the invites will "typically" be valid for 24 hours. And in a bit of good news for those waiting, once the invite has expired it will be recycled and sent to someone else. OnePlus had also said those with an invite will be able to invite other users. The invites that can be given away are expected to be valid for 1-2 weeks.
The initial invite availability will be limited, both in terms of invites sent out by OnePlus and of invites that users can pass on to others. OnePlus mentioned how availability will increase as production ramps up. If you remember, the production schedule was recently adjusted to have the 64GB model coming before the 16GB model and it looks like the "first real batch of invites" will roll out in late May (for the 64GB Sandstone Black version).