Installation
From the module README file:

• Enable the Organic Groups module. If you want to protect some posts so that only certain users may view them, enable the 'Organic Groups access control' module as well. Please make sure og is working well on its own before enabling other OG related modules.
• Visit the admin/og/og page.
• On admin/og/og, see the content types table at the top. Click edit beside each type to set its 'usage'. You usually want to create a new node type via admin/content/types page and then designate that node type as a group from here.

  This module supports designating any type of node to be a group. This node type should be defined by a custom module or via the admin/content/types page. When defining your type, you usually want the title label to be 'Group name' and the body label to be 'Welcome message'. Since all nodes of this type are treated as groups, you will usually not want to designate the standard page, story, or book node types as groups. The capacity to make custom node types groups means that you can have custom fields for your groups and even several different kinds of groups.

  On the admin/content/types page, disable comments and attachments for node types which are designated as groups. Click the edit link beside those types.

  On the admin/build/themes/settings pages, in 'Display post information on' section, uncheck each node type which has been designated as a group.

• Set other preferences on admin/og/og as desired. It may take some experimenting before you arrive at a configuration well suited to your needs.
• On the admin/build/block page, enable the 'Group details' with a low 'weight' value. Optionally enable the other 'Group' blocks.
• Grant permissions as needed on the admin/user/access page
• Begin creating groups (visit the node/add page), joining those groups, and posting into those groups. The join link appears in the Group details block, for non invite-only groups.