The Microsoft Popfly team has sent out invitations for the alpha program to all of those who requested them as of June 19. Those who haven't received one yet are urged to request one again. It is possible that the acknowledgement e-mail sent out after requesting an invite will never arrive if one requests an invitation using a Windows Live ID that doesn't have an active email address associated with it. As well, the e-mails can be marked as spam and the users will never receive see them. Popfly invitations come from a human's Hotmail account but a lot of people have spam filters turned up pretty strictly and Popfly invitations are being classified as spam. So if you haven't got one yet, head over the site and request one again.