Yes, you can certainly tell people that you are participating in a playtest or preview. However, you cannot show them the game or discuss NDA-covered topics with them. This includes, but is not limited to: bugs, feedback, gameplay elements, or game content. These types of information are reserved for our closed forums. Please review the full NDA & Guidelines.
Feel free to encourage others to join the Age Insider program so they can have their own chance at participating in preview and playtest opportunities.