In the Manage tab on your Project home page, you can specify which email addresses will receive automated notifications. Notifications will generally be tied to processing events in your project, such as a pipeline completing.
Emailing Project Users From the XNAT UI
All users with access to your project are listed in the Accesstab, along with their email addresses. At this point, there is no project-level console for sending an ad-hoc email to all members of a project from within XNAT. There is such a console at the site-wide level, which XNAT Administrators can use. See: Notifications and Alerts in XNAT