If you have a message you need to send to volunteers who have RSVP'd for an event, you can do so from the event. 

Go to your Calendar > click on the event > under Volunteers > choose Contact Attendees.

From here, you can customize the message you want to send to attendees. You can choose if you want to message all of the volunteers who have RSVP'd or if you only want to message a single shift. 

You can also choose to enable text messaging if your plan allows.