what does it mean that they cannot see the dashboard? They can see the framework (labels, title, etc) but cannot see the data (database access issues?) or cannot see the workbook in the list @ tableau server (tableau access issues)?
first check that server installed machine is accessible in all world wide.
Have you accessed in same environment or else accessed in outside of your company too.
Check the server config window.
Select whether to open a port in Windows firewall. If you do not open this port, users on other machines may not be able to access the server.
they cannot see the framework as well as the data