WordPress 3.0+ has made the activation of a multisite fairly straightforward, and the documentation on the codex is clear & exact…

there is no reason to try to improve upon it…  just go straight here: http://codex.wordpress.org/Create_A_Network

Once this is done multiple sites can be run off of a single deployment, and while everything is being run off of a single database the import/export function works in a clean an reliable way.

Plugins & Themes are imported via in the “master” portion of the site. Plugins can be selectively enabled, and themes need to be uploaded | activated in the “master” | applied to each individual site

Membership to a site can be made selective to a particular site, and the full range of privileges is available to choose from.

BUT BEWARE: best practice is to create the site with the same info used for the “master” login, if you use a different main login identity for the sub-site, then you won’t be able to be a visible member of the site with the “master” identity (not listed as author, etc.)

http://wordpress.stackexchange.com/questions/10599/how-to-change-the-main-site-url-on-a-multisite-installation-network

How to change the main site url on a multisite installation (network)?

A very extensive search & replace (with regex if needed) in the entire db. Yes, everything. Yes, it’s a pain.

For those of you without the required sql knowledge. The steps below can be used to change your main site url variable on a network installation.

Assumption:

Windows Operating system
MySQL Admin basic tasks
WinGrep

Steps:

Download mysql dump.sql of your full wordpress database
While using wingrep find all matches for @url then replace with @newURL
Upload the dump file with the new variables replaced
Change your wp_config.php file to the newURL

You are done!

Lines 83-89

define( ‘MULTISITE’, true );
define( ‘SUBDOMAIN_INSTALL’, false );
$base = ‘/wordymulti/’;
define( ‘DOMAIN_CURRENT_SITE’, ‘plasterdogproductions.com’ );
define( ‘PATH_CURRENT_SITE’, ‘/wordymulti/’ );
define( ‘SITE_ID_CURRENT_SITE’, 1 );
define( ‘BLOG_ID_CURRENT_SITE’, 1 );