Social Loafing is the phenomenon of a person exerting less effort to achieve a goal when they work in a group than when they work alone
According to Karima Mariama-Arthur, there are 3 ways to prevent social loafing. One simple strategy to prevent it is to keep the group small. Another strategy is to develop the rules of engagement. Set up ground rules at the beginning of the assignment so each member will be aware of their responsibility and cause less conflict. One final strategy you could use is to assign separate and distinct contributions for every team member. Each team member would have their own task and have to rely on themselves to get their individual tasks done, which will make sure each team member picks up their own slack.
Sources: www.en.wikipedia.org/wiki/social-loafing
www.blackenterprise.com/career/3-steps-discourage-stop-social-loafing-team-building-leadership-tips