Ok, I am really dumb at these message boards

If you click on someone's name on the left you'll be taken to their profile. From there you have a couple of options. You can send them a message or add them as a friend.

Sending them a message is just what it sounds like. It's a little internal email system here on the site and only they'll see your message.

Choosing Add as Friend will send them a friend request (which they will then have the option to confirm or deny). Once you're friends with someone, their updates will appear on the newsfeed on your home page. They have some choice about what they publish in those updates. It could be anything from letting you know when they've completed their food diary or how many calories they've burned today to more personal updates that they choose to post.