When registering multiple competitors using Smoothcomp, you have to register each competitor individually. There is currently not an option for individuals to register multiple competitors, checking out once instead of multiple times.
Coach/Manager accounts can do this if the students are listed under their account (instructions can be found here https://support.smoothcomp.com/article/86-how-to-use-your-coach-account).
The Smoothcomp website has an excellent support page with answers to many of your questions. You can visit their support page here, support.smoothcomp.com.