Add to Circle. And if they also add you in their circle, then you two become friends. That's the rule in EP. Add to circle is like "follow" in twitter and if the person you added also added you to their circle, then that's how they become your friends.
Do the other person get a notification when you add them to your circle?
Right below their profile photo there is a "add to circle" link. That is how you add them.
it's the Add to circle it's the same as adding to friends :)