There are some option to achieve this job
1. Single Sign On Multiple Domain
Single Sign On Multiple Domain module allows login to multiple drupal
websites from a single website. If you have multiple drupal website
installed at either on same server or other server, then you just need
to login to one and it will login you to other websites as configured
in the configuration page. For this you have to install this module in
all website and do required settings.
This module also allows to create account for user if user do not
exist in other website. It also allows you to synchronize user role
while creating account.
2. Multisite Login
Multisite Login allows a user to login to all sites in a multisite
configuration, even though the multisites do not share domain names.
For example: politicker.com politickernj.com politickerny.com
NOTE: if you are using several sites on the same domain (ex.
politicker.com, nj.politicker.com, ny.politicker.com, etc.), then you
do not need this module. The multisite functionality in Drupal core
has the ability to share logins for sites like these without
This module was designed to be an alternative to the singlesignon
module but using a technique that does not block search engines from
accessing your websites.
3. OpenID Provider
This module implements an OpenID Provider in Drupal based on the
OpenID Authentication 2.0 specification. It turns your Drupal site
into an OpenID provider and allows users of your site to login to any
other OpenID-enabled site using their Drupal password and identity.
According of my experiences I suggest first try Single Sign On Multiple Domain module