Add columns to the Members List CSV Export.

my_pmpro_members_list_csv_extra_columns.php

<?php
/*
	Add data from the pmpro_memberships_users table
	The pmpro_members_list_csv_extra_columns passes an array of columns.
	The keys of the array are the column headings. The values are callback functions to get the value for that row.
*/

//add the column to the export
function my_pmpro_members_list_csv_extra_columns ( $columns ) {
	$columns["startdate"] = "my_pmpro_members_list_column_a";
	
	return $columns;
}
add_filter( 'pmpro_members_list_csv_extra_columns', 'my_pmpro_members_list_csv_extra_columns', 10 );

// Callback to get the start date column value
function my_pmpro_members_list_column_a( $user ) {
    global $wpdb;

    // Query the 'startdate' for the specific user
    $startdate = $wpdb->get_var(
        $wpdb->prepare(
            "SELECT startdate 
             FROM {$wpdb->prefix}pmpro_memberships_users 
             WHERE user_id = %d 
             ORDER BY id DESC 
             LIMIT 1",
            $user->ID
        )
    );

    // Format the date if it exists, otherwise return empty
    return $startdate ? date('Y-m-d', strtotime($startdate)) : '';
}