At MyHeritage family sites are run by Site Managers who invite others to become family site members. Persons who are neither Site Managers or invited members can view public family sites (including the family tree), but the information that can be viewed is limited. If you find a family site you are not a member of but would like to view the full family tree on, you will need to request membership.
You can do so by clicking on the ‘Request membership’ link, while visiting that site.
Private sites at MyHeritage only allow visitors if they were invited by the Site Manager. If the site that you wish to become a member of is a private site then it will be closed and you will only see a login page, but you can still request membership.