You can send an email to an individual VolunteerHub user by clicking on the ellipsis button for that user from the Users page and choosing 'Email User'. This will email the user through VolunteerHub.
To email them from your own email client:
Use the search button at the top of your site to search for the user you'd like to email. Click on the name of the user you would like to email from the results provided. On the user's profile page, click the email address to open a new email to that user via your default email program, or copy and paste the email into the To: field of a blank email.