Allowing members into groups
Using Jamroom
one of the objections to releasing it was from me. I get
"(Someone you don't know) has invited you to join (some site you've never heard of)" emails all the time.
I know in the case of some of them the "click here to unsubscribe" is actually collecting my email address so I end up on more lists I don't want to be on. So my usual move is to tag it as spam and let gmail's spam algorithm add that site to their list of 'suspicious'.
Too many people do this and you'll find your site ends up not being able to deliver legitimate email like new users signup application emails.