Is it legal to have regular floor employees help HR go through other employee's open enrollment forms to see if they have made changes? All of our personal information is on there?

It may or may not be legal. But it is totally unethical.

They can make any employee a "temp" HR helper if they want to. What personal information is the concern?

There are no real Federal laws (in US) their may be state laws, but this would be no different than them hiring temp workers from a agency, outside the company.
(I guess strangers looking at your personal information is better?)

HR is just another department in most companies, and subject to the same manning policies as all the other departments of the company. Not at all illegal or unethical unless there is more to this story than written.