This tutorial will show you how to push out an online assignment to a learning group as opposed to the entire class roster.
From wherever you want to build your assignment, go ahead and click "Assign."
Once you're on your assignment settings page and make all your assignment options, scroll all the way down to the bottom of the page and deselect any of your sections.
Then move to the right where it says "Filter Learning Groups" and use the drop-down menu to select the learning group or groups that you would like to push the assignment out to.
Finally, select "Create Assignment."