Use an id on a button to update_post_meta in post

As @s_ha_dum has stated, your syntax mixes JS and PHP; and Ajax wil be needed. I’ll try to point you in the correct direction.

This should be your javascript:

jQuery(document).ready(function($) {
    $('button#rsvp_button').click(function qwe4_tracker(){

        var data = { action: 'my_action' };

        $.post(ajaxurl, data, function(response) {
            //alert('Got this from the server: ' + response);  // Uncomment to use for testing
        });
    });
});

NOTES:

1. We use jQuery(document).ready(function($){ to begin our JS. This is to prevent clashes when using the global $ jQuery variable.

2. The var data contains the action; which must be used in the PHP add_action described below.

And this should be your PHP function to handle the AJAX:

add_action('wp_ajax_my_action', 'my_action_callback');
function my_action_callback() {

    global $post;  // You still may need to pass the POST ID here from the JS ajax.
    // get each post by ID
    $postID = $post->ID;
    // get the post's custom fields
    $custom = get_post_custom($postID);
    // find the view count field
    $views = intval($custom['number_attending'][0]);
    // increment the count
    if($views > 0) {
        update_post_meta($postID, 'number_attending', ($views + 1));
    } else {
        add_post_meta($postID, 'number_attending', 1, true);
    }
    die(); // this is required to return a proper result
}

The PHP function will handle the updating of the number in the post meta.. and the JS tells the PHP (via ajax) to update when the button is clicked.

NOTE:

You may still have an issue getting the proper Post ID. If so, then you are going to need to declare this in a global JS variable via your PHP code.

Good Luck!