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Nearly one month ago Microsoft announced the details about the preordering of the Developer Edition of Hololens and there came the Queue Of many doubts and questions about the Invitation process. Yesterday Microsoft tried to answer many of their questions via a forum and here’s what it has:

We are inviting developers to purchase in waves, as soon as devices are available. If you have applied, check for an email from us, indicating the status of your application. If your application is approved, a wave number will be provided within this status email. Each wave will ship over a period of two to three months. Timing may vary. When your wave is being served and your device is ready, we will send an email invitation to order. To secure your device, please order within a week of receiving your invitation to purchase. If you miss this window, you are still approved and can purchase, subject to device availability.
Please keep in mind, each wave will ship over a period of two to three months. Most folks who will be part of Wave 1 have
not yet received their invitations to order, so do not panic.

If you received an invitation from Microsoft you can contact them from here.