How to add Organic Groups after Site has been created
It should not be necessary to re-create the whole site from scratch. After installing Organic Groups, you can mark an existing content type as "Group Content" if you want the nodes of that type to belong to a group:


Go to admin/structure/types.
Click on the "Edit" link next to the type you want to modify.
In the Organic groups tab, check the Group content checkbox. It will create a Groups audience field in your content type.


After that, you will need to assign each node to the appropriate group, in the Groups audience field.