Help required to set up multisite using organic groups (OG module)
Ok I have done it using Organic groups, spaces module and PURL. 


First Install Spaces module after installing drupal
Then you can install PURL module and then organic groups module and OG theme module. OG theme associates theme to a group. You can also install OG menu if needed.
Once PURL is installed, go to admin/config/search/purl/types, enable subdomain option and domain option if needed.
Now create a content type of Group type or install OG example module (it automatically creates a organic group content type)
Now create a node which is of type group - node/add/group
Fill in the options and enter the Subdomain. For the subdomain to work you should have added ServerAlias *.site.com in your httpd.conf file
Once the node of type group is created your minisite is ready.
Using OG theme you can associate a separate theme for your organic group created.
Using og menu you can associate a separate menu for your organic group.