Signing up users on a multi-site blog

It appears that the admin of a site, on a multi-site network, has no way to allow users to register for their specific site? The wp-signup seems to default to the main site in the multi-site network.

Is there a plugin that will solve this issue? I imagine that many developers use multi-site to set up client sites, and this would be a deal breaker as their clients would not be able to register users (sign them up on the front end) of their website.

Thanks in advance for the feedback.