1)Turn editing on in the class
2) Edit the settings on the topic
3) In 'Restrict access' settings click Add restriction
4) from the options in the pop-up choose 'Group'.
5) Make sure the settings are set to 'Student must match the following' and choose one of your groups. (It is also possible to use the opposite statement, must not to exclude a group)
In this case the 'eye-con' 
has no line through it. This means that the assignment will be visible to everyone in the class, but there will be a message to explain it's only accessible to those in a specific group.
If you click the 'eye-con' a line will appear 
. This denotes that the assignment will be completely invisible to anyone not in the group.
6) Click Save and return to class
