You can do this by going to the relevant Event screen, and adding a message to the wall using the "Post & email community" option on the dropwdown list. (Or just "post to wall" if you do not want an email sent out and just want to publish it on the wall.)

Click the POST button, and all members of the community will be emailed and addressed individually. The event details (title, description etc) will be included in the message, as well as a link to the conversation on the event message wall so they can reply. They can also click the event link to RSVP or volunteer directly for tasks. This is how the email will look:



However if you only want to message your event volunteers, you can instead send a group message to the helpers on a particular task or activity by selecting the "Post & email helpers" option before clicking POST. This will email only the members who are helping already. The cool thing is though that as more people sign up to help, replies to this conversation will dynamically add them to the recipient list for emails going forward. Neat, huh? Anyone no longer wishing to receive replies can also "unfollow" the conversation. Best of both worlds :)