The Woodbury Army is a massive group of armed Woodbury citizens rounded together by The Governor to kill Rick Grimes' group in order to capture the Prison, as well as to satisfy his own hatred of the group.
A Herd, also known as a Horde, is a large group of zombies. They are usually attracted by sound, leading a few individuals to come together, eventually finding and merging with new groups, growing larger and larger and even more ferocious.
The Saviors are a large group of antagonistic survivors who were mentioned in "A Larger World" and first appeared in "Something To Fear". The Saviors use a large factory surrounded by a wall of impaled and tied zombies as their base.
The Kingdom is a community of survivors found in Washington, D.C., who are based at a high school surrounded by a wall of buses and sheets of metal. It is the second biggest settlement after the Hilltop Colony.
The Whisperers are a mysterious group of survivors who disguise themselves with roamer skin to blend in with them and not get noticed. They are first introduced in Issue 130. They serve as the primary antagonistic group in Volume 24: Life And Death.