keep doing the water changes. The tank probably isn't finished cycling yet. I made that same mistake a couple weeks ago with a small guppy fry tank. I had to change 50% of the water every single day for close to two weeks until the tank finished cycling. I have never heard of salt doing anything for nitrite levels, I just heard it can help prevent some bacterial infections.
My guppies tended to die one by one as well, not all at once. It seems the weaker ones go first and then as the condition persists or worsens, the next weak ones go until the strong ones are left. One fish acting badly is a warning.
This may or may not apply, but I found that I had better luck with only three or four guppies in a ten gallon than say 6 or 8. As soon as I added more than four a bunch died.