Will/Jem.
We don't know what this is going to lead to, right? Gathering our forces might be null and void if all we need to do is leave and immediately return. This seems more like precaution than anything else, but I understand if this is a bit much. I always want you at my side, William, but I will do this for the both of us.
And, to be additionally frank--though I appreciate the offers given and know that Mina would be safe and well taken care of with others, I would feel insurmountably more comfortable with this situation knowing that she is in your care. I try not to speak for Tessa when at all possible, but I feel deeply confident saying that she would feel the same way. There is no one we trust more than you.