getFrom() method should return the sender of the message in the format Some Person <someone@somewhere.com>. If you need just the email address portion of the string, you can extract it using regular expression like this (adapted example from getFrom() method description):

var thread = GmailApp.getInboxThreads(0,1)[0]; // get first thread in inbox
var message = thread.getMessages()[0]; // get first message
Logger.log(message.getFrom().replace(/^.+<([^>]+)>$/, "$1"));

That regex replace the string returned by getFrom() method with the part of the string in angle brackets (the sender's email address).