There is quite easy possiblity how to register even without invitation code.
As you are trying to register to wordpress site, just leave invitation code blank. After submiting, you'll receive error 476. However, user is already created.
That rises two major issues:
1. without proper antispam protection, database will be soon flooded with fake registrations
2. with plugin such as WP-United, which links users of WP and phpBB, you can easily reset password in forum and gain access to WP too, completely avoiding need for invitation code.