You can click on that person's name and then edit contact details, type in their email and press ok.
next top left corner of the window you can see share button, still click on the same person then use the share button on top left. By default while sharing you are giving that person the right to edit, if you want you can uncheck the "allow (user) to edit family tree" if you just want that operson to view only.
You can also do it without editing contact info, just click the person you wish to allow and then press share (top left) and the enter the email manually on this screen. again you have the option to allow that person to edit or just view, same process.
Hope it helps
Mhaziq
