First time i make a question out of a Novice area
Is possible create a user, that can access to just 1 dashboard and nothing else?
Go to apps, pick a dashboard, and within each dashboard there is a "share link" called Dashboard Direct Links.
Send that person the link (probably the cloud one would be best?) and that will be the only thing they can access. It will be viewed within a browser, not the Hubitat app.
Alternatively, password protect all but the one dashboard that you want them to access. If they don't know the password, they can only open the non-password protected dashboards.
There is a setting that you also might want to select in the Advanced option at the bottom of each dashboard setting page. (That section is where to set a PIN by the way). The option of "Lock down dashboard so no changes can be made in the web ui of this dashboard" is probably a good idea.