Anyone who has submitted a design which was then selected to become a Threadless tee is automatically an Alumni Club member.
Anywhere an Alumni Club member's name is displayed, it will be lavender like this. Also, look for the mini shield icon next to their name in the forums and comments, as well as the larger icon next to their picture on their profile page.
Simply put: once you're in, you're in. You will only receive one Alumni Club new member's package, regardless of how many times your designs are chosen.