To add someone to your board's GovernorHub page, click on the 'Governing Board' or 'Trust Board' tab (this is normally the 4th tab).
Click on the 'Add Member' button to bring up the 'Add a member' window
Fill in the person's email address, and hit 'Next'. Then fill in their name and (optionally) their membership details.
When you've input all their details, click on the "Invite to GovernorHub" button.
This sends them an email with a link to set up their account and get signed in.