Is there a way to customize the confirmation email that JotForm sends when someone signs up for an event?

Yes, you can edit it using the Email Editor. Here's a guide:

In Form Builder V4, click the "Settings" menu and then go to the "Emails" section. Select the Autoresponder email and then click the "Edit" button.

In the Email Editor, you can customize the confirmation email content. Click the "Save" button when you're done.

Hope that helps. Thank you.