Two things -
1) you have a WordPress.ORG install on your own host and you are not hosted on WordPress.COM (see below for more info)
2) You have a Register option in your sidebar in the Meta Widget - anyone can click on that and register - not sure what they register for but the option is there (some sites require registration to comment) -
The site you are asking about does not seem to be hosted on WordPress.COM so you need to make friends over at WordPress.ORG the keepers of the software you are using.
This site is for support of sites hosted on WordPress.COM. You should address your questions to WordPress.ORG the keepers of the software you are using: http://wordpress.org/support/
For more on the difference: http://support.wordpress.com/com-vs-org/
If you are asking for help with a WordPress.COM site then we need a link to that site to give you accurate help.