I found a strange meaning of sign up here ,It says that sign up is to enlist, like in military.
Also I found this "Sign up" in business domain:
By signing up early adopters (i.e. those quickest to adopt or most influential in your space) you can make it near impossible for new entrants to get any foothold
Have I figured it correctly? is it means to enlist customers?
Can it be that all sites around are saying "sign up" in that sense?