New users registered on my site?

I run MyLastNickel. I have a question and I am not sure where to put it so hopefully this is an appropriate place. Tonight I had five new “Users” register within a thirty minute time frame. This might not sound like much but read on. The total number of users is six and I am one of them. I have the site set up to register new users as a “subscriber”. To be honest I am not really sure what this means.

What really bothers me about this is that I can’t see any new visits to the site through the site stats on WordPress, nor can I see any new visits to the site through Google Analytics. So how did they register as a user on the site without actually “visiting” the site? Should I be worried about someone hacking into the site (not sure why anyone would want to), or is this just another form of spam? If it is some type of spam is there some type of security settings that I can change to help prevent this in the future, or am I just being overly cautious here?