You can configure event reminders to be sent to volunteers 2 days before, 1 day before, or right after the event.
Sign in >> Log Hours >> Event Sign-ups >> Notifications (at bottom of setup page) >> check which notifications you would like to send to your Volunteers.
However, at this time there isn't a way to send out customized emails to specific volunteers. If they provide their email during sign up you can copy their email address into your emailing provider and send messages that way. During initial sign up for a shift the volunteers can send the organizer a message that will show up in the Roster of Volunteers that everyone can see.