If I buy a 24 or 48 hour pass, would I be able to use this with my other two travel companions (ie we all enter and exit the metro with the same pass)? Or do we need to purchase individual/separate passes?
A pass is personal and can only be used by one person. If it worked the way you describe, they would sell only one pass a day and entire population of Milan would be that guy's buddy.
Now I have a Ventra Card for use in Chicago and can use it for others traveling with me, but it is a loaded-value card, so every time I scan it, it deducts a fare. But that is a horse of a different color.