If you write your surname on ancestry family trees you will find a list of family trees with that name. when I find something that might connect my family however, I do not see a link to contact the person who posted that family tree. have others had this problem?
When you click to view their tree, their name should appear in the top left corner. Click on their name and the button "Message" should appear. However, if they choose in their privacy settings "no contact", that button will not appear.