Premium users can choose to share selected monitors with other ChangeTower users. Once a check is shared, users will see check notifications in their dashboard, and will also begin to receive email notifications (if enabled for a given shared check).
Invited users who don't already have a ChangeTower account will receive an email invitation asking them to sign up with ChangeTower. Once they accept that invitation, they'll immediately be able to see notifications for your selected monitor(s) -- and, if enabled, will also receive email notifications for those monitors.
You can add a colleague / client to your account via the Manage Team Notifications section. Once you've done that, you simply need to pick which Monitors they should be added to: