Basic usage of SageMathCloud is free but before you can start using it, you need to create an account. This can be done at https://cloud.sagemath.com/
It is strongly suggested that you, and your students, sign up using your institutional email addresses. This makes it much easier to find students in the system.
You don't need to worry about getting accounts for your students ahead of time.
Once you've created your course in SageMathCloud, you just a add list of student email addresses to it. All students immediately get sent a course invite, and when they create an account with that address, they are good to go.