You can give manage permission of whiteboards to non-admin team members from role settings (Settings > Access Permission > Roles). Admin users will always get access.

Give access permission to clients from client permissions settings (Settings > Client portal > Client permissions). Enabling this, you can share whiteboards with them.

Then the permitted users will get a new menu item in the left menu. Labeled as Whiteboards.

Note: If the user's left menu or default left menu is already edited, then the Whiteboards won't be shown by default. Then you can add the menu manually from the left menu settings.