Click on the share button its inbetween the share and my account. A side panel will come up by default the To: line is set to whoevers box you are on when you click share. So if you are on Jane Smith's box it will say To: Jane Smith. If in the contact tab you have someones email adress that email will show up by default in the To email: If you don't it will be blank and you just have to type it in. However you can just change either the To: or To email: and put in whomever your sending it too and their email address.
Another option you can just directly email them a link of the tree. This is located at the bottom of the share pannel.