Many family site members help Site Managers work on the family tree by adding names and photos from their side of the family. That way Site Managers can grow their tree and family site members can make sure the information on their side of the tree is up to date.
However, allowing members to edit the tree is at the discretion of the Site Manager. The Site Manager can configure settings to allow or disallow editing the family tree for family site members.
If you could once edit the family tree in the family site you are a member of but now you are not able to edit the tree anymore, please check with your family site manager whether settings have been changed.