Hi Noman,
Add users to Site Visitor group so they will be having read only access. Go to Site Settings --> Peoples and Groups --> and select visitors group in the site.
Users you will add to any group, will have site access. If user is not added to any site group or granted individual permission, will not be able to access the site.
However public site of Office 365, everyone will be able to see as this is a public site.
By default I think only public site has everyone as visitor access.