(I don't know about the others, as they have disregarded the instruction to give us the URL to which they are referring)
You do not have a blog hosted on WordPress.COM. Therefore, you are in the wrong forum. We will not be able to provide useful answers for us, as our software is not the same. Random people cannot add themselves as users to our blogs here. You need to be at WordPress.ORG instead for support.
This is extremely common with WordPress.ORG blogs, particularly those running older software. There used to be some sort of exploit or something they could do, but if you are running an up-to-date version of the software this should not be a problem for you. If you';re not, update. And you can just shut off the ability for people to add themselves as users.
For specifics and all other support, please go to WordPress.ORG