Internal and external people that are not set up as users of intelligentcontract can receive Alert notifications. However, because they are not able to login to intelligentcontract, the Alert email they receive gives the option to Snooze the Alert only. They can opt out of receiving the notification altogether (this is a UK law to offer this. You will see if a person has opted out on their person record - it gives the date they opted out)
If you set up users as read-only users, when they receive the notifications, they will have a [Review] and [Complete] button, so they have a bit more control.