Is it possible to resend an email to a contact list but only to the contacts that have joined after I sent it first?

Yes you can. Here's what you do: 

1. Select the newsletter and send it as normal. 
2. You can then use the "Filter" option and "Filter by date subscribed" This will give you a date field that you can select. 
3. You select the date that you want to filter by, so it would be something like: Contacts that have subscribed after 20 Oct 2007. 
4. Send the newsletter as per normal. It will only be sent to those that have subscribed after a particular date.

This will only send the emails to people who have subscribed after you have sent the email the first time.