BuddyPress.org

To add; should anyone else be looking to do what I am (without the else tag), here’s how to do with so that when the user is on the REGISTER page, the tab is highlighted:

<?php if ( !is_user_logged_in() ) { ?><li<?php if ( bp_is_page( BP_REGISTER_SLUG ) ) : ?> class="selected"<?php endif; ?>><a href="register">Sign up!</a><?php } ?>