If you click on any living person in your tree, one of the options in the upper left is to “Invite (name) to share family". Once you click on that, you can email them directly from within Family Echo, or copy a link and send it to them some other way.
You can also choose to give them permission to edit the tree or just to view the tree. You do this with the checkbox "Allow [name] to edit family". Changing this checkbox also changes the link option.
NOTE: Once you share your family tree, there is no way to remove either the view or the edit permission. Even deleting the family tree will not change their ability to access it. You should probably download a backup copy of your tree before you share it.