Social Loafing is one of the problems faced by group work. It means that the bigger the group the less effort some people will make, which has a deterimental effect on the output of the group.

"The answer to social loafing is motivation. A competitive environment may not necessarily get group members motivated. For Rothwell, it takes "the three C's of motivation" to get a group moving: collaboration, content, and choice.

1. Collaboration is a way to get everyone involved in the group by assigning each member special, meaningful tasks. (CSCW, 2000) It is a way for the group members to share the knowledge and the tasks to be fulfilled unfailingly. For example, if Sally and Paul were loafing because they were not given specific tasks, then giving Sally the note taker duty and Paul the brainstorming duty will make them feel essential to the group. Sally and Paul will be less likely to want to let the group down, because they have specific obligations to complete.
2. Content identifies the importance of the individual's specific tasks within the group. If group members see their role as that involved in completing a worthy task, then they are more likely to fulfill it. For example, Paul may enjoy brainstorming, as he knows that he will bring a lot to the group if he fulfills this obligation. He feels that his obligation will be valued by the group.
3. Choice gives the group members the opportunity to choose the task they want to fulfill. Assigning roles in a group causes complaints and frustration. Allowing group members the freedom to choose their role makes social loafing less significant, and encourages the members to work together as a team."

http://en.wikipedia.org/wiki/Social_loafing

It maybe important to consider these motivators when designing the system to ensure the maximum number of people participate.