You can create local accounts for them with one of your users who has the right to do that. You only need to be able to navigate to the Account_overview page.
If you deploy your app on cloud or sandbox – you should have a link to them. You can share this link with others.
Depending on whether security is turned on or off they might need credentials to login to the application.
Mendix account will be needed if you want the users to access the app via studio (pro) or the developer portal – or also via Mx SSO (though I’m not sure about the last one as there were some changes)
Hope this helps!