Steps
Open the board you want to share
Click the "Share" button (purple) in the top-right corner of the toolbar
In the Share Board dialog, enter the email address in the "Add people by email and press enter" field
Select their permission level from the dropdown (default is "Can View")
Click the purple "Invite" button to share the board
The recipient will receive an email invitation to access the board
Use "Copy Link" at the top-right of the dialog to quickly copy a shareable link
Click "Publish as a Template" at the bottom to make your board available as a template
Notes
You must have Editor, Admin, or Owner permissions to share a board
Different roles have different access levels and capabilities
You can add multiple people to the same board by repeating the process
Changes to board permissions take effect immediately
The "Who has access" section shows all current collaborators on the board
You can also add users directly from your workspace using the "Add user from workspace" link