You can write:

$(document).ready(function() {

    $("#select-all-teammembers").click(function() {

        var checkBoxes = $("input[name=recipients\\[\\]]");

        checkBoxes.prop("checked", !checkBoxes.prop("checked"));

    });

});

Before jQuery 1.6, when we only had attr() and not prop(), we used to write:

checkBoxes.attr("checked", !checkBoxes.attr("checked"));

But prop() has better semantics than attr() when applied to "boolean" HTML attributes, so it is usually preferred in this situation.

ref: https://stackoverflow.com/questions/4177159/toggle-checkboxes-on-off