"Send to a Friend (STAF)" or "Forward to a Friend (FTAF)" are very good ways to get your message across to readers that are not on your list. However, you should not add these email addresses to your list! It's just not good email practice and in some countries it's even illegal to do so. These people have never opted-in or gave you permission to email them. If you do add them to your list, you'll be hurting both your brand (you'll be perceived as a spammer) and your deliverability rate (you'll see a spike in spam complaints).
Chad White was recently hit with such an email from Sony at an address where he’d never been a subscriber but had received a Sony email from a friend who used Sony’s send-to-a-friend (STAF) functionality. That forwarded email arrived more than 6 months ago. Read what he has to say about the subject.