need help to limit access of external ssh-users

Hi, I have limited experience in linux but I'll find my way around if kicked in the right direction, I hope. ;) I'd want to open a ssh port to my homebox for external staff and some friends. I don't suppose they'll try to be funny but safe is better than sorry. For this I created a group "external" and userids like "staff01". How can I keep them from walking around in my system ? They should only be able to drop and collect documents which are in their home-directory. I have sshd working and login works, so I only need advice for further security aspects. My system is an up to date SuSE 7.2 Thanks Andreas