Joining Spore Illustrated is FREE and EASY. Simply look for the Join Now button on the upper right side of the page.
It looks like this:

Once you've joined, we encourage you to fill out your profile, by clicking on your "My Profile" link and clicking easy edit (from there a template will load to help you figure out what to add to the page).
then

After joining, you can become friends with other members and send them messages or compliments by clicking on their names. You are also encouraged to create, edit or add to pages on this site to make it the best Spore site available on the Internet.