Can you double check if you have used the correct port 2222 to login to Sandbox? (Sandbox runs inside docker container hence the SSH port is mapped to 2222)
# ssh firstname.lastname@example.org -p 2222
Enter Password: hadoop
Or use the Http Based WebClient terminal to login to Sandbox:
After using the correct SSH port you should be able to use the hadoop commands and should be able to see all the users.