If I’m behind on my coursework, should I still attend Group Crits?

Yes, absolutely! Even if you have nothing new to present, you are still required to attend as attendance will be taken by the facilitator. You also need to participate, so use this time to practice offering constructive critique! 
Also, if you’re feeling stuck on your coursework, being a part of discussions during Group Crits can help you learn from how others approach design problems. You never know what inspiration and ideas you might encounter on the next call!