JSFiddle - React, Tailwind, and code Playground

HTML

<ul id="group-input-names">
    <li id="Test1"></li>
    <li id="Test2"></li>
    <li id="Test3"></li>
    <li id="Test4"></li>
    <li id="Test5"></li>    
</ul>

CSS

#group-input-names {float:inherit; list-style-type:none;}

JavaScript

$(function() {

    //I would use jQuery Map to build Array
    //http://api.jquery.com/map/
    var ids = $("#group-input-names li").map(function() {
        return $(this).attr("id");
    }).get(); //Get Required to convert to regular javascript array 
    
    console.log(ids);
    
    var invites = {}
    invites.users = ids;
    
    //Then use JSON.stringify() to pass array to server
    if (ids.length > 0) {
        $.ajax({
            type: "POST",
            //url: "",
            data: JSON.stringify(invites),
            cache: false,
            success: function(html) {}
        });
    }

});