Navigate to your course and ensure the "Edit" button is on.

Click on the assignment you want the restriction for. In this example, the teacher duplicated the original assignment and is now restricting it so only one class has access to it. This could be useful for different classes/periods, modified/adapted/EAL students, etc.

Choose your group out of the options you already set up.

Set of 1 restriction(s)
Student 
Restriction type
must
must not
 match the following
1: Group restriction
Group
Choose...
(Any group)
P3
P4
Add restriction...

Now only that group has access to that task.

 Not available unless: You belong to P4 (hidden otherwise)

How to Restrict Access to Tasks using Groups