I want to use one user account for a large group of people--how do I prevent them from editing the display name or changing the password. Thanks.

Cathal, either it has been too late in the night or you mixed up the editor screen when multitasking, but IMO your answer does meet the question.

Unfortunately, the first 3 words of your answer are correct, there is no option to prevent a user from changing his password, which would be very useful e.g. for Demo Accounts. I suggest to log this into the feature request list in ROadmap and into the public issue tracker at support.dotnetnuke.com.

You can, however, use "user settings"  in Admin :: Users to populate Displayname automatically, e.g. with FirstName + "  " + Lastname.