Introduce yourself to the group and prompt the others to do the same. Just because you are not meeting face-to-face does not mean that you should detach yourself from the group. This can be done over email, or in a chat room; whichever is easier for the group as a whole. Tell the group who you are, what you do, and why you are taking that particular class.
Set goals and deadlines that are clear. Students that are working online have a tendency to procrastinate. This can easily be remedied by setting guidelines and making expectations clear to every member of the group.
Keep track of the deadlines. Though no one wants to be the group disciplinarian, does not seem to be pulling his weight, call him on it.
Keep in regular contact with the other members of the group . If you are having trouble holding up your end of the assignment, discuss it with the group. Make sure no one else is having trouble.