Activity generators are like neurons (in fact in the code they just are neurons which implement a special interface), except they don't respond to inputs, but only produce outputs. They are thus activity or output generators. They are represented by squares rather than circles in the GUI.
Activity generators behave almost identically to neurons so for information on how to build and edit them see the building and editing docs.
For example, to create an activity generator go the Network > Insert menu and select activity generator.