Allow users to register
As the code currently stands, you can either enable or disable invite codes in your installation. This is done in etc/config.pl:
$USE_ACCT_CODES = 1;
By default, this is set to enabled in a new installation. If you're developing, you may wish to set this to 0. This will allow anyone to create an account without having an invite code.
To create and distribute invite codes (so you can make accounts while $USE_ACCT_CODES is on), log in as the system user. Go to the Admin Console ($SITEROOT/admin/console) and type:
make_invites <username> <count> <reason>
make_invites system 1 "to create my hacking account"
Your invite codes will print out on the success page. They will also show up at $SITEROOT/manage/circle/invite.bml (where they can be emailed if you'd like).
Or, you can create a 'promo' invite code. Make sure you have the "payments" priv, then go to $SITEROOT/admin/invites/promo and click 'Create new'. If you do it that way, you'll only have to remember the name of the promo code you set and enter that for account creation.
Notes on config files
There are many possible locations for config files. The quick and dirty method is to edit the ones in $LJHOME/etc/. The better method is to copy them to $LJHOME/ext/local/etc/ and edit them there. It helps if you know which one you're editing.