From Stanford CSD History
To create a new user, you must be logged in and your account must be designated as a 'sysop'.
1. Go to Special:Userlogin, when logged in as a sysop. 2. Click on "Create an account" to get to the account creation form. 3. Enter a username and an email address, and click the "by email" button. Note need $wgEnableEmail=true or else the sysop must pick a password and send it to the user. 4. The account will be created with a random password which is then emailed to the given address.
It may be appropriate to edit the text displayed when a non-user attempts to log in. This can be done at MediaWiki:Nosuchuser, when logged in as a sysop. Use plain text without any special formatting, as the formatting is ignored and the text is literally rendered.