Setup membership levels and with a bit of minor tweaking you can set all new customers to a particular level, say you created "Pending" and "Authorized" to the membership levels. You then would just need to modify include/register.php to make the default level be "Pending" so that you can easily change them to "Authorized" via the admin. You would need to setup all of your products and categories to only allow for "Authorized" members to view and use it which by default, x-cart can do.
hth.
