Mikasa would have attended if only to just further understand her world now. She probably would have been observing most of what she could see and understand, just trying to piece things together in her mind. She's probably worried about security since if things can come through the Tesseract, what's stopping "bad" things from coming through.
She probably would have volunteered herself to help out or to train anyone who wanted to learn how to fight. She would be willing to let someone look at her 3D Maneuver Gear, which basically allows her to "fly" through places with high buildings or trees as long as she could attach the strings to something (think like Spiderman moving through the city, that sort of thing), if it meant that more security could be added.
Mikasa would also be concerned about finding a way back home, if anyone has done research on this yet.