You can message any instructor directly from their profile page, using the button at the bottom of the booking form. All your messages are archived so you can reference them at a later date. Also, any time you contact an instructor, he or she receives an email notification (and vice versa).
You can continue any previous conversations from the “Messages” tab in your dashboard. There's a box in the upper left corner which allows you to “Select a conversation”. If you've booked lessons, your instructor's phone number will be provided here as well.