With Seesaw, when it comes to adding new classes and teachers, that is a job for IT support.
When it comes to making changes (adding/removing students, changing names) to existing students in classes, that is the job of the specific class teacher (possibly with assistance from a DLC).
To make changes, just click on the spanner/wrench icon in the top right then click Manage Students, then type name or type the student ID (eg mchug12345), you may find searching using the student ID more successful, as that can't be changed, but their name may well be different...
If you use the ID Seesaw will locate the student, just click Add Student to confirm:
If you don't know the student ID and can't be bothered to retrieve it from CIMS, then just type in the student name again, choose to add the student, and then Seesaw should locate any student matched for you to select from.
Removing students is similar, but easier, just click on the student's name, then click where it says, 'remove student from class'.