Each event in Athlinks has a Start List that shows Athlinks users who have opted to join the list. Being in the Start List doesn't register you for the event, but it's a way for you to show the Athlinks community which events you're going to be participating in. Friends who have Athlinks profiles can see events whose Start Lists you've joined in their "Friend's Upcoming Events" section on their profile.
To join a Start List, just find the Event in Athlinks using the search feature, then click on the Join the Start List button on the right side of the About page.
A window will pop up asking you to supply the Race Distance you'll be participating in, your goal time, and a brief message to fellow racers and friends in Athlinks. You'll be added to the Start List once you've filled out the information and clicked Submit.