You can post a question on volunteer requests. 

1. Click on the Class name in the left navigation tray
2. Click on Signup in the Updates tab
3. Click on the volunteer event
4. Scroll down to see the Signups||Comments||Invitees tabs
5. Tap Comments
6. Enter comment/question in the comment box at the bottom
7. Click Send

For more information, check out the following articles:

Requesting Volunteers for My Class Event

Signing Up to Be a Volunteer
Canceling My Volunteer Sign Up
Asking a Question on a Volunteer Task
Assigning Parents to Volunteer Tasks