Is it legal to define rental fees based on a number of occupants?

[Linda Roberts]

Is it legal in Massachusetts to define rental fees based on a number of occupants?

It definitely make sense for me, because I pay a water bill, but I don't usually see it for the long term rentals. Today I found one listing that specified rent per a number of people. I wonder if I can do the same or I'll put myself in a trouble. This listing was by owner, so he may not know all the rules.   

$3200   for 3-5 people.

$3250  for 6-7 people

 $3350  for 8 people.