Possible robot sign ups

Hi,
I recently added the multisite/multiuser script to my site. Since it was started, I have had a number of signups. Much faster than I would have believed possible. Especially since I haven’t even announced it to my existing membership.

My suspicion is that it is robots. How did they find me so fast? How do I stop it?

I have better wp security and have tried to initiated capcha, but have only would up with more problems.

Can anyone out there help me sort through this mess and stop the robots?