Usergroups are a way for you to create groups of users, based on some criteria that you find appropriate. You don’t have to use usergroups—in the default phpBB installation, no groups exist—but this can be a useful feature.

Every group has a moderator. This community member is responsible for managing the group, and is not to be confused with a forum moderator. Group moderators have no special privileges over the forum topics or postings; they have special privileges only for the group they are moderating. They can add and remove members from the group, can accept or deny membership applications and can change a group type.
What types of groups exist? A group can be open, closed, or hidden.
An open group is visible for everybody and accepts new members. In order to become a member to the group, a user applies to join. The group moderator can accept or deny applications. He or she can also add members to the group without having those members to apply first.
A closed group is a group that does not accept applications. The group moderator can still add new members.
A hidden group is like the closed group, but is not displayed to the users. Only logged-in members of the group can see it. The group moderator adds new members.

3. Click the Create New Group button.
4. Fill out the form as shown:

5. Click Submit.
You’ve successfully created a new usergroup called Test with the world-famous user The Dude as a moderator. This group is open and accepts membership applications.
If you now visit the front-end of the board, go to Usergroups, and view the information about the Test group, you’ll see a screen like the following:

Because you’re still logged in as an administrator, you see the group moderator options for changing the group type and adding/removing members. You don’t see the membership applications interface, because no applications currently exist. Also, because you are not a member of this open group, you see the Join Group button.
Click the Join Group button to apply for a group membership. This sends the group moderator an e-mail advising that there is a new pending application. The moderator logs in, goes to the usergroup (the screen above), and accepts or denies this application. If the application is accepted, you become a member of the group and get an e-mail communicating that fact.

4. Change the name and the description of the group as shown. Also, change the Group status to Hidden group.

5. Click on Submit to save the changes.
You have successfully edited the group. It was a simple thing to do, just like creating the group. But this time you had two additional checkboxes on the Edit group form: you could delete the group, and when changing the group moderator, you could select whether the group’s ex-mod stays in the group as a regular member or is removed from the group.
The administrator has the option of setting group permissions. The group moderator doesn’t have control over them. The group permissions work just like the user permissions. The only difference is that you can set a permission type once per group and it will apply to all members of the group. This can save you a lot of work. Although a member is part of a group and has group permissions, you can still override these permissions on a per-user basis. This means, for example, that you can specify that all members of a usergroup can post announcements, but then forbid this for a specific user of the group.
Group permissions are accessible through the Permissions link in the Group Admin section of the Administration Panel navigation menu:

One common task you might want to do is to create a special hidden forum that can be used only by certain users (for example, a forum for moderators, where they can talk about board or user-related issues without those discussions being publicly available).
In order to do this, you:
Create a Private [Hidden] forum.
Create a private group.
Give access to the group to use the private forum.
So far we have discussed the first two points. We’ve created a special private hidden forum called The Dudes forum and you’ve a hidden group called The Dudes gang. Now the last point: giving The Dudes gang access to The Dudes forum.

5. Change all dropdowns on The Dudes forum row from OFF to ON.
6. Click Submit to save the permission settings.
You’ve granted privileges for the whole The Dudes gang usergroup to be able to read, post, vote, etc., in The Dudes forum. Here are a few more points for you to think about and experiment.
You can use the Advanced Mode for granting permissions by turning ON/OFF selectively; it’s not necessary to turn all ON or all OFF.
Simple Mode cannot be used for setting group permissions, not in the phpBB version this book is based on.
Forum moderators and group moderators are completely independent of each other. The Dude is a group moderator of The Dudes gang, but is not a forum moderator in The Dudes forum.
You can make everybody in a group a moderator by setting the group’s Is Moderator status.
Group permissions are a fast way to set permissions, but you can still use the user permissions to be more specific. User permissions override group permissions.
You can have several usergroups with different access to a forum. For example, you can set up an open group called "Junior Dudes" that can only read The Dudes forum, while the good old The Dudes gang usergroup can still post.