The tendency of individuals to put in less effort when working in a group context is called:

Social Loafing.

In social psych, this is a phenomenon that occurs when a person exerts less effort (than they do working by themselves) assuming the group they're working in will pick up the slack. Social loafing is characterized by

the free-rider effect: a person puts works less because the group will pick up the slack

and

the sukker effect: previously hard worker group members lower their efforts in response to the "free-rider"