Since the programme has changed i have had 2 invites one in august and the other invite just on tuesday
this is what it says at the bottom of the email i hope this helpe you
We have a lot of Power Users but only a limited number of free samples. In order to distribute the samples more evenly, we will send invitation emails with a list of available samples to select groups of Power Users in rotation, Power Users can then claim one of the available samples to test. Every Power User will have the chance to test our products. If you have not received any product test invitation emails for 6 consecutive months, please directly email email@example.com with the subject line “Power User - Product Waiting List Request” to be placed on the test waiting list.