Lauren: DON'T GO.
As much as it pains me to say it, Lucy is right. This is a trick. The letter says they used Gina for the Ceremony, but they couldn't have. Gina's blood was contaminated by the Hart Study. She would've killed the Elder.
I know what you mean when you say that you would rather die for Harry's cause than die for the Order. But you don't have to choose.
Harry didn't have Jonas. There is no way he could've rounded up all those people. And that letter couldn't be from Ryvest because he would know about Gina's blood.
This is a trick. Don't fall for it.