In the section for the general settings, there are 2 settings you need to check if you want to keep and be able to view “expired” signups:
Make sure that you have the “Automatically clear expired signups?” option UNchecked, to make sure those signup records are not deleted from the database after the event has passed.
Then, if you want to see that info on the ADMIN side, you need to CHECK the option that says “Show Expired Tasks?”
Note that if you require your volunteers to have a WordPress user account and be logged in to sign up, then their personal info is saved via the regular WordPress Users database table. My plugin only keeps track of information they fill in on the sign-up form.