Method 1:
SQL Query
SELECT USER_DISPLAY_NAME
FROM ubbt_USERS
WHERE USER_REGISTERED_ON >=1222560000

This works based on the date. You can calculate that date on this site.

Method 2:

I would prefer this one. Just make a note with each newsletter of the highest USER_ID and use this query:

SQL Query
SELECT USER_DISPLAY_NAME
FROM ubbt_USERS
WHERE USER_ID >5000


[Linked Image from siemons.org]