DK, you're in grad school, but working on a group project with undergrads? You guys are grouped as equals?
If this is the case, you simply report their behavior to the professor/instructor and he/she should re-arrange things accordingly or he/she should pull the other students in for a nice, friendly chat.
If the prof or instructor refuses to do either one of these things, you need to find out why. But I'm guessing he/she will take reasonable action. And if he/she doesn't, well, then you may have to speak to the department chair if you genuinely feel you can't work in this situation satisfactorily.
