Nature, you wrote -- But the problem is that, some times our group members can not work properly. Then what can we do?
1st of all try to form a group with known persons (if possible) rather randomly selecting members. Consult with all the group members about the assigned topic. Divide the topic into some sub-topics and distribute those among the members according to their interest and capability. Fix a time for collecting a rough format from them. Then, try to find out the possible positive outcomes from the rough format with the help and suggestions from them. If any one fail to do his/her task. Try to motivate him/her for doing his/her task, give some suggestions (if possible). If s/he again fails, try to find out the obstacles behind the failure and try to solve it. If s/he willingly deny to work in the group, cleverly do something to take some extra efforts from him/her regarding the other task of the group work.
It'll be better, for forming a group next time excluding the member/s who were willingly deny or ignore his/her responsibility. Otherwise it may hamper the success rate of the group and can create negative result in some aspects as well.