The Community Hub has a 'Get In Touch' section at the very bottom of the page (the footer). This is for linking your social media accounts, email address, and physical address.
To edit and add contact information, go to Settings in the main navigation, and open the Organization Info tab. Select the box 'Show on Hub' to publicly display the field in your Hub footer.
When a volunteer clicks the 'Contact Us' button, their desktop email client will open with a new blank message addressed to the contact email you've provided.