Why don't you just create bunch of users, assign all of them the same password (which you can hardcode into your source-code) and let them just fill in their usernames? That would give you the functionality you want. A single value login so to speak.

Edit: just to be blatantly obvious: this is a bad idea and I recommend just giving them usernames/passwords to remember.

I'm with driesvints. It isn't a good idea. If you want to reduce mental effort maybe enable OAuth for Fb/G+/etc login?

I agree this is a bad idea, I would use their email address as username (they should know this) and you can then use this to reset their password when they forget it