Click on Chris (for instance), and then click on "Invite Chris to share family" on the left. From there you have a number of options. Note that there is no way to unshare a family tree. Even if you delete the tree, any one with whom you have shared access will still be able to access it. If you are going to give them editing rights, I would recommend you download a backup copy of your family tree first. Really, you should probably do that after any major edit.
