Is there some way to auto-fill information once someone has signed up for an event?

Auto-complete, which is on by default, allows a volunteer to just enter their email address when signing up for a slot.  The rest of their information, including any custom fields, is automatically filled.  This also works from the Administrator’s side, by entering the email address of a volunteer, the rest of their information will be filled in.

Also on the Administrator side, you can add volunteers from your Database. To learn more about the Database, go here.