Ok, I think I've got it! It's an environmental problem. Cron doesn't know where to find the commands so I've used full paths and it seems to work.
Code:
#!/bin/bash
/usr/sbin/groupadd someuser
/usr/sbin/useradd -d /home/someuser -s /bin/bash -m -g someuser someuser
echo "someuser:somepassword" | /usr/sbin/chpasswd