This is why you drop the subject that forces you to do group work
In all seriousness, if you don't want some crappy marks, you're mostly likely going to have to do the whole assignment. This isn't junior school where you can afford to give no two craps about assessments.
I guess you could always imply that you have to carry your group to the teacher, but I wouldn't recommend this because of the nature of the assignment. Group assessments imply working as a group and some teachers may subtract marks (look in your marking criteria?). Since you know the level of work your fellow classmates are going to display, perhaps you could allocate ~simpler~/easier tasks to your group members? Basically, break down or chunk the workload for them. That way at least they're doing something and although you may end up doing 90% of the work, you're not doing 100%? Think of them as the primary sector and you're the tertiary sector in an economy employment thing.
On the bright side, group works (at least in my school) aren't that hard. Usually it's just research rather than actually using your brain.
Either way, good luck!