Same user for multiple sites I am planning to use different server for multiple purpose.
like 

Groups.eg.com

Privatecontent.eg.com

review.eg.com

etc...
I have up to 5 different subdomains and everything runs in different server and different drupal installation.

I want to use the same user base across all these sites.

I am aware that if a single drupal installation have multiple sites we achieve this using domain module.

Is there a best solution for my use case.
Because we are going to load 10k users in the site.
Sounds like Bakery module would meet the requirements for you. It will handle single sign-on between Drupal sites on different sub-domains.

As far as performance goes, Bakery is used by drupal.org when you switch between https://www.drupal.org, https://groups.drupal.org, https://assoc.drupal.org and others. 

What it doesn't do is to switch between different domains like drupal.org and drupal.com. Bakery also doesn't sync user information, if you have user fields.

For more advanced features like SSO between Drupal and non-Drupal, look at CAS module, or some of the other single sign-on solutions.