The buddy/block button is the one with the icon of two people, one that has a star and one that has a red slash.
To add some as your buddy, click on the Who's here button, click on their name and choose if you want to be their buddy. This way also works for blocking people.
When someone is your buddy, you'll be able to tell where they are and what world they are in. If someone asks to be your buddy, you can say yes, no or later.