Someone fowarded my private email invitation

Found another weird activity going on my spreadsheet, someone is sending my private email invitation to some people and I read names and email addresses that are not part of my list, this is for an upcoming event that I'm involved in inviting guests to attend and do coverage. I wondered where it came from and who sent my customized email invitation to some attendees that I invited.

This is the 2nd time that I encounter it, the 1st one is just a minor case, but seeing more sign ups who are not invited are very alarming already on my side, but I want to thank them for passing the message and referring these awesome people with cool blogs who are interested to join with us for that event. I just hope they emailed me personally and show their interest in being part of it, there's nothing wrong in sending me an email, a Facebook stat message or even a poke :)
don't worry guys.. I don't bite. Feel free to message me if you are interested to join with us.

It is also my fault for not reminding them that this email is confidential and can't be forwarded to anyone, my plan is to remove those unwanted sign ups, but I'll include them on the next event, I need to prioritize first the people I personally hand picked and invite for this event, I mix and match the different personality of people and hope to have a fun result in the end of the show. Too bad that I won't be there to witness the grand show, but I'll join them next time.

The situation is good x bad x good, it is good on both sides but I need to teach and remind them that forwarding private email invitation is a big no.