Mike,
I am a partial plan holder (flex plan) and I got an email, put my name, account number (it's on your invoice), and number of people attending and it said that it went through so I hope it works! Also I don't see why they would entice me by sending me an invitation to something I'm not invited to. You should get an email from them soon and please say when you do so I know that it isn't a fluke that i got one.
Here is the link to register from my email!
Open House Registration Form