Yes, each BoB input has to have its own resistor. The resistor pulls up the input to 5v (logic 'high') as its' normal state. When triggered the proximity switch pulls the input to 0v (logic low), so Mach3 has to see the input as 'active low'. Without the resistor you have a 'floating' input and the voltage on it could be anywhere, so it would not be predictable or reliable, and probably not work at all.

It is possible to configure switches as groups, but unless I was struggling for input pins, I wouldn't go there unless I had to.

Cheers,

Rob-T