Training Offerings (Allow users to express interest)

Summary

How do you use the "Allow users to express interest" for the Training Offerings?

Content

I am trying to allow users to sign up for a classroom training that has not been scheduled. After I get enough people, the instructor will schedule a class. I am trying to use Training Offerings and the catalog and search widgets to accomplish this but I can't find how someone "enrolls" or adds themselves. 

Has anyone does this? If so how? Thanks.