Duplie can allow you to easily send Thank You emails to your volunteers after their sign up.  To enable the reminder, go into your config (on admin dashboard and then action configuration).  You will need to update the config Send Thank You Emails to Yes.  If this is enabled, we will show you reminders on your admin dashboard.  They will be grouped by date and the event.

You can also just send out a Thank You email for a single event without being reminded about all of them.  To do this, go into your event that has passed and under action choose Send Email/SMS.  When the modal opens, make sure to choose the permission Thank You.  You can also include a volunteer survey (if you have one created).  By including a survey, we will send a link to your survey form and auto login the user.  The survey submit will be stored on the volunteer's profile under submitted forms.

The default Thank You text is customizable.  There is a config called Thank You Email Text that you can update to fit your needs.