You need permission to manage users to share dashboards. The Admin role has it.
Open the dashboard and select the Dashboard actions menu.
Select Manage access.
Under Add a user or Add a role, pick who to share with.
Choose Read to let them view it, or Write to let them edit it too.
Select Save.
[Image: The "Manage access" dialog with a user and a role added]
People only see the properties their own access allows, so two people can see different numbers on the same shared dashboard.