Multi-user notifications will notify multiple users at once when an incident is triggered or escalated, and can be configured within an escalation policy. To notify 2 or more users at once, you will want to configure an escalation policy so that multiple on-call schedules and/or users are in the same escalation level.
Using on-call schedules for multi-user notifications
An important concept to grasp is that only one user can be on-call per schedule. A natural first instinct would be to add multiple layers to a schedule, however the way PagerDuty's system works, the lowest layer on a schedule takes precedence over any others.
The following screenshot demonstrates what will happen if multiple layers are stacked on top of each other (Layer 3 takes precedence over Layers 1 and 2):
Configuring your escalation policy to notify 2 or more users at once
To notify 2 or more users at once, you will want to set up an escalation policy with multiple on-call schedules/users on the same escalation level.
Go to Configuration > Escalation Policies.
Click New Escalation Policy to create a new escalation policy, or click an existing escalation policy's gear icon and then Edit to edit an existing one.
Click in the field below Notify the following users or schedules and select the on-call schedules and/or users you want to notify when an incident is triggered or escalated.
Click Save and you're done!
Note: If you have Live Call Routing enabled to connect directly, PagerDuty will not call all users on an escalation level — only one user per escalation level will receive a call. If an incident triggers via Live Call Routing (if either no one answers, or you've selected "Leave a Message"), however, all users on an escalation level can expect to receive notifications via their configured notification methods.
Choosing an on-call schedule or a user for multi-user notifications
Select a user if you always want that specific user to be notified when an incident is assigned and/or escalated to an escalation level.
Select an on-call schedule if you only want to notify the user from that schedule who happens to be on-call when an incident triggers.
Limitations for multi-user notifications
Notifying too many users at once runs the risk of creating confusion about who owns an incident. With this is mind, multi-user notifications are limited to 25 users or schedules on Standard and Enterprise plans and 10 users or schedules on Lite and Basic plans.
This Knowledge Base article addresses best practices about when to notify one user vs. many users.