I’m fairly sure you can’t retrieve your number. When you join something like this appears at the top of the page: Members: 1,002, Threads: 2,838, Posts: 23,459
Welcome to our newest member, Jack Francis. This indicates that Jack is member 1002. This will be superseded by the next new member and the number lost. Please correct me if I’m wrong anyone.
Go to the users profile then move the mouse over any of 'Send xxx a Private Message!', 'Click here to email xxx' or 'Search for all posts by this user' and you'll see in the status bar the user id, or actually select to send them a PM and you'll see the number in the address bar.