JSFiddle - React, Tailwind, and code Playground

HTML

<form>
    <select name="users" id="theSelect" multiple>
        <option value="">Select a person:</option>
        <option value="1">Peter Griffin</option>
        <option value="2">Lois Griffin</option>
        <option value="3">Glenn Quagmire</option>
        <option value="4">Joseph Swanson</option>
    </select>
</form>

JavaScript

document.getElementById("theSelect").onchange = function() {
    var len = this.options.length;
    var selected = [];
    for (var i = 0; i < len; i++) {
        if (this.options[i].selected) {
            selected.push(this.options[i].value);
        }
    }
    alert(selected);
    //getUsers(selected.join(","));
};