Family Echo isn't setup to do exactly what you want. The idea is that you are sharing the tree with someone who is on the tree, and you would want the tree centered on them to make navigating easier for them. But sometimes you want a more generic link to share with a group of family members. I do that by picking a reasonable common ancestor, clicking that they are living and sharing their link (make sure you uncheck the box that says they can edit the tree). After you copy the link, kill them off again.