wp-signup link for a new site installation

Let's say I have 2 sites running on a multi site installation using the folder method instead of subdomains.

Site 1 - the original multi-site. Let's pretend this site is about all animals in general
http://www.site.com

Site 2 - A site soley about dogs, also running on the multi-site installation.
http://www.site.com/dogs

I would like users to be able to signup for a new site at Site 2 about dogs and not be signed up to site 1 which is all animals. I figured the signup link would be http://www.site.com/dogs/wp-signup.php but that just redirects me to http://www.site.com/wp-signup.php.

Is this because I'm using folders instead of subdomains?