If you create a container named Composites at the root of your outline, and then create composites within that container, and, finally, give the composite a name, then that composite becomes a master. Your personal master composites will appear in Note > Create Composite and you can create new instances of those composites by choosing one from that menu.
To name a composite click the composite on a map view and choose the pencil icon
Note that the if you create a composite based on a master, and then change the master, the instances are not updated.
You could add an agent to a composite, but then you could end up with lots of instances of that agent. Which might not be a good idea.
For more info, see the various articles on composites, special attributes related to composites, composite operators, and composite-related actions at aTbRef