One method is to use the site creation wizard (see also getting started tutorial), but instead of using the default, set the Community Site Root to a location within your existing site.
For example, if your site is at /content/my-company/, and you wanted the community at /content/my-company/my-community, you would fill out the Site Template values as :
Community Site Root : /content/my-company
Community Site Name : my-community
The home page for the community would be http://localhost:4503/content/my-company/my-community/en.html
- JK
