Instruct the students from day one regarding the rules of any group homework time. Let kids know that the time is only to be used for work, and not for chatter. Establish ground rules at the outset so that children know that violation of those rules will have accompanying consequences.
Establish that the group succeeds or fails together. Create stakes for the group so they are encouraged to facilitate quiet work with each other rather than becoming loud.
Have a routine planned that children become used to for working in groups. Assign this time at the start of class, for example, for children to work together to finish group work. The schedule and regularity and built-in expectation for a certain behavior helps keep the group quiet and focused.
Create a system of positive reinforcement for groups who spend this assigned time working well without disruption. A fun activity after a certain number of classes for good behavior, or some sort of posted recognition, helps kids to feel as if they're achieving something by behaving while working in their groups.