I see you can make friends, but how does that work?

"Your Virtual Garage is more fun if your friends are on it too. To find out if your friends are already members, you have to log in first and then go to the 'friends' page. In the right column you will find three blocks. The top block lets you invite friends per email. Select an appropriate language for the invitation email and add some personal text to identify you as the sender. Use one email address per invitation, and just click the send button. The second block allows you to find your friends on Your Virtual Garage based on their name, , granted they either use their real name or a familiar alias. The last block shows all open friend invitations that you have received. You can simply accept or decline each of the friend requests."

I don't seem to be allowed to invite my friend by email. It says he was already been invited.

"That is possible. You may not be the first to invite an individual to Your Virtual Garage. You may want to search by user name to find this friend and send an invite from his or her profile page. If the friend has not accepted the first invitation yet, we will not allow additional invitations to clog up his or her email box. You can of course choose to use traditional email to invite the same friend."