Method 1:
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:
SELECT USER_DISPLAY_NAME
FROM ubbt_USERS
WHERE USER_ID >5000