They have to sign a confidentiality agreement, agree not to spill any information before the official product announcement, and delete all "unannounced HP product information, including postings made by participants in the website or forum within 48 hours of the posting" or risk being kicked out of the evangelist program (which means no hook ups on reviews) and face possible legal action by HP.
So when I got an invitation from Badoo, which bills itself as Europe's hottest social-networking website, to attend their American launch in SoHo a few weeks back, I hesitated before hitting the delete button.