Tell the lecturer, asap really. Similar things happened to me and the only reason I survived was because the supervisors each time knew about the problems.. Obviously don't make it seem like it's only you who has the problem with the others ;) Pointing out the extra member trying to push her way in, and they'll probably sort that out themselves, lecturers are surprisingly good at making it seem like you've said nothing to them, they've just noticed a problem on their own ;)

Basically once the lecturer knows, even if they can't do anything about it yet, do what you can and make sure you can show you've put the work into it. When they flounder around and can't show they've worked as a group with you and haven't done much work themselves, then it's obvious what happened, the lecturer can step in, and you'll generally come out of it with a suitable grade.