Warning

WordPress Multiuser is not for everybody. Those wishing to install the software must realize that it is server farm software on the level of what runs WordPress.com, blogger.com, and other large multi user blogging sites. Users should have basic knowledge of PHP, MySQL, how to edit files, and how to review error logs in case of trouble. Experience with the regular Wordpress platform as well as the knowledge on how to install addons such as WordPress Themes and Plugins would be a requirement as well.
It is not for beginners.
[edit] Installing

Below are some tips and steps that should help you get a copy of WordPress MU installed and running on your domain.
[edit] Requirements

Things needed for this process:

1. A domain or subdomain. (e.g. domain.com)
2. A domain using DreamHost's PHP5 cgi (this is important)
3. Patience

[edit] Step One: Getting everything set up correctly

Before we can even get to the installing of WPMU, we need to have our domain happy with a wildcard DNS entry. To do this, you will need to contact the support team and request that this be set up for you. Just let them know that you are installing WordPress MU and need a wildcard DNS entry set up for the domain you're installing it on.
[edit] Step Two: Installing WPMU

1. Download a nightly build.
2. Read the included instructions within the compressd file.
3. Create a MySQL user and database for the install. Note the information for later use.
4. Uncompress the contents of the downloaded file to your local computer
5. Upload the contents of the wpmu-date subdirectory to the root of your web space.
6. Reset the webspace's main directory as well as the /wp-content subdirectory to 777.
7. Load up the site within your browser and follow the detailed instructions.

[edit] Troubleshooting

If things go wrong, try looking up your problem on the Official WordPress MU Forums. If you can't find an answer, ask it there. Be sure to note the Debugging page for needed information before posting to the forums.

• Make sure that you have installed the software without the 'http://www' in the domain name.
• If you run into trouble, the first place to look is within the server's error logs. You may find a clue as to what is occuring. Also note that WPMu creates it's own error_log within the main directory.
• Be sure to search the forums with keywords covering your issue. Chances are someone has already run into the issue that you are having.