1. create your site and subdomains.
2. upload q2a to all of them
3. install one of them as parrent site
4. config others to use 1st one's User Table as it's own user table(in config.php constant QA_MYSQL_USERS_PREFIX should be set to 1st ones table database prefix and it should use another value for QA_MYSQL_TABLE_PREFIX)
5. visit subdomain, click Instal and your done
one which I made was immihub.com(and even external site immihub.co.uk which is networked with same site) which was magicaly stable and never had any problems about it's networking.
read more in your config.php file, here is some description from there:
Set QA_EXTERNAL_USERS to true to use your user identification code in qa-external/qa-external-users.php
This allows you to integrate with your existing user database and management system. For more details, consult the online documentation on installing Question2Answer with single sign-on.
The constants QA_EXTERNAL_LANG and QA_EXTERNAL_EMAILER are deprecated from Q2A 1.5 since the same effect can now be achieved in plugins by using function overrides.