Just keep track of the visible elements in an array:
var visibleEls = [];
$('.myclass').bind('inview', function (event, visible) {
if (visible == true) {
// element is now visible in the viewport
if(!~visibleEls.indexOf(this)) visibleEls.push(this);
} else {
// element has gone out of viewport
var i=visibleEls.indexOf(this);
if(~i) visibleEls.splice(i, 1);
}
$('body').toggleClass('red', !!visibleEls.length);
});
Note that you could simplify it to
var counter = 0;
$('.myclass').bind('inview', function (event, visible) {
counter += visible*2-1;
$('body').toggleClass('red', !!counter);
});
but this could be more error-prone.