1)You'll have to install all boards into seperate directories and preferrably in the same database, using different table prefixes (I will use master, slave1, slave2, ..., slaveN).
2)Designate one board as the MASTER install, this one will act as the user-database for all other forums. The rest will be called SLAVES from now.
3)For consistency, rename the MASTERS shared tables to not use the prefix (using a tool like phpMyAdmin f.i.):
CODE
ALTER TABLE master_users RENAME users;
ALTER TABLE master_user_group RENAME user_group;
ALTER TABLE master_groups RENAME groups;
ALTER TABLE master_user_group RENAME user_group;
ALTER TABLE master_groups RENAME groups;
4)Delete the unused tables of the SLAVE installs (for each slave repeat this step):
CODE
DROP TABLE slave1_users;
DROP TABLE slave1_user_group;
DROP TABLE slave1_groups;
DROP TABLE slave1_user_group;
DROP TABLE slave1_groups;
5)Change for all installs (MASTER and SLAVES) the entries in includes/constants.php to (remove the $table_prefix):
CODE
define('GROUPS_TABLE', 'groups');
define('USER_GROUP_TABLE', 'user_group');
define('USERS_TABLE', 'users');
define('USER_GROUP_TABLE', 'user_group');
define('USERS_TABLE', 'users');
6)You're done. All your forums are now using the same user database. Banning and configuration options can be set for each forum seperately. If you want people logged in on one forum be logged in in another forum, all you need to do is make sure the cookie settings in the admin panel are the same. All forums have to use the same domain, but they can differ in subdomains (f.i. forum1.mydomain.com will have mydomain.com as the domain for cookies).
Original Link: http://www.phpbb.com/community/viewtopic.php?t=12057
I.P.Board
To make two IPBoards use the same users table thuss making a user registered on more boards at the same time you need to use the I.P.Converge setting found in:
Tools&Settings -> I.P.Converge Configuration (11th option in IPBoard 2.3.3)
More information on how to use the Converge Settings are found in the documentation of the board:
http://www.invisionpower.com/community/boa...umentation.html
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[size="6"]VBulletin
For VBulletin i am sorry but i can't make a good tutorial because they require a license to view their codes.
If you are licensed and want to merge two boards under the same user table check the following link:
http://www.vbulletin.org/forum/showthread.php?t=98233
Hope this helped someone and if i can find more tutorials to merge users on different pages like joomla etc i will post them here.
Cheers.