XNAT has an email notification service which can be employed to notify some or all project users when a specific event takes place, such as an image session upload or a pipeline process completing.
If you have trouble with this functionality, be sure to check with your XNAT Administrator to make sure that Notifications and Alerts in XNAT are enabled.
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.
All users with access to your project are listed in the Access tab, 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