Hi, Peggy . . . If you're asking how to easily send a reminder email to all attendees at one event and you want the email itself to include their ID numbers, yes, there's an easy way--I do it with my events all the time. Go to your Events List and at the far right of each event, you'll see a TOTAL column. Click on that number & up pops your list (note this includes canceled ones as well). Check boxes in the first column for ones you want to send the email to (or the box at the top of the column to select all), then "Send Email" button at the bottom. Within your email, you can include merge fields like the ID field (it's under Personal dropdown list) or any other merge field you want.
Another cool feature is that you can imbed any reminder emails within the set-up of the event itself, so you don't have to manually go to this list & type your email from scratch. Go to Edit Event, the Emails tab, "Schedule an email . . ."
Good luck! You're gonna love regonline . . .
Karen