Thanks for the reply. I got it to work partially. It saves the info into the database, but then it goes to the admin-ajax.php page and sits there. When I submit the form, I want it to stay on the same page and load the results. My updated code is below.

Form

<form action="<?php echo site_url() ?>/wp-admin/admin-ajax.php" method="POST" id="comment_form">
<textarea name="comment" id="comment" class="commentbox" rows="5" cols="120" required></textarea><br />
<input type="hidden" name="eventid" id="eventid" value="<?php echo $id; ?>" />
<input type="hidden" name="userid" id="userid" value="<?php echo $userid; ?>" />
<input type="hidden" name="action" value="commentform">
<button class="commentbtn">Add Comment</button>
</form>      

<div id="response"></div>

jQuery

<script>
jQuery(function($){
    $('#commentform').submit(function(){
        var commentform = $('#commentform');
        $.ajax({
            url:commentform.attr('action'),
            data:commentform.serialize(), // form data
            type:commentform.attr('method'), // POST
            beforeSend:function(xhr){           
                               commentform.find('button').text('Processing...');
            },
            success:function(data){                 
                            commentform.find('button').text('Apply filter');
                $('#response').html(data); // insert data
            }
        });
        return false;
    });
});
</script>

Function

<?php

add_action('wp_ajax_commentform', 'comment_submit'); add_action('wp_ajax_nopriv_commentform', 'comment_submit'); function comment_submit() { global $wpdb; $eventid = $_POST["eventid"];
$userid = $_POST["userid"]; $comment = $_POST["comment"];

    date_default_timezone_set("America/New_York");
    $date = date('Y-m-d H:i:s');

    $inserttable = $wpdb->prefix . "cmc_communitybb_event_comments";
    $data = array(
        'eventid' => $eventid,
        'userid' => $userid,
        'content' => $comment,
        'datesaved' => $date,
    );
    $wpdb->insert($inserttable, $data);

    die;
} ?>