Sorry, but there is no built-in function for sending mass emails to volunteers.
The contacts for each sheet should also be getting an email when any one signs up for their sheet, so they could simply collect those email addresses as they receive them in their email client, and then easily put them all in a CC or BCC field for a mass email.
Or, as you said, you can export the sheet data to CSV and collect the emails that way as well.
Feel free to add that to the many feature requests for the plugin on my site, as that’s not a bad idea, but there are other considerations:
Are you then giving the contacts for each sheet special access to the admin area where they could fill out a form to mail to all volunteers? In our PTA, we were not comfortable with letting all committee chairs have access to the WordPress admin area. So, if you don’t do that, then you need to create some kind of special page/form on the front end, and give each chair person special access to that page (would have to be password protected), plus all the code needed to determine who has access to what, and which volunteers from which sheet they are trying to email, etc…. Much easier to just collect the emails manually and let them send any mass emails the way they are used to with their own email client software.