Up to this point, you already have a dashboard to which only you have access, together with administrators of your Organization. The security of access to the content is still to be configured, that is, the space used, the page created and cards included in the page.
To do this, use the "Access settings" option in the context menu of the selected space.
Access settings in a shared space
Among all options you can implement, the most direct one might be to check the "Apply same permissions to children" option in the space access settings dialog, to apply in chain inheritance the same settings as indicated in the space to all pages and cards in the space.
With this you can configure which users and groups of users can view, edit or manage the content.
Remember that in addition to being able to apply security to access content, you can also apply security to access data, using the Data Policies in datasets.