Multisite Drupal with Organic Groups problem.. custom /user and /user/register links for each group? I am using Organic Groups for as a solution for Drupal Sub sites, it is working perfectly, even themes are switching via Organic groups theme but I am having problem with /user/register and /user pages.

I want to pass context to user pages like user/groupname/register or user/groupname , so how can I do it ? because /user currently throws me out of the currently active theme for Organic Group. 

Also one more thing, the Group Id which I want to pass should be as alias?like pathauto paths.
I would probably look at PURL for this, an API to make a part of a URL persistent.
PURL itself does nothing since it's an API, but you can get started with OG Purl.
Be warned, it's more advanced to be creating your own bridge like OG Purl does. A lot of motivation is required :)