You can set KC to charge by weight for boarding.  You also have the option to set multipet rates.

First, click on File, System Options, Boarding tab.

Check the box for "Charge by pet weight"

There is also an option called "Show adjust run rate prompt".  Let's say you have rates setup for your runs.  If your pet's weight class charge is $45 but you choose a run with a different rate, you can turn on the prompt so KC asks which rate do you want to use.  Here is what it will ask.

If you only want KC to use the weight class, do not check 'Show adjust run rate prompt'.

Hit Apply/OK.

Next, click on File, Lists, Weight Classes.

Hit New Weight Class.

Enter the weight range, 1st pet rate and the multipet rates (2nd - 5th pet).

Hit Save after each one.

For this to work, each pet must have a weight on his pet card.

Let's say your client has 5 pets with weights 12, 53, 63, 68, 72,

Your weight classes are:

0-25

26-55

56-80

81-115

Pet 1 at 12 lbs will be charged the first pet rate for 0-25 weight class.

Pet 2 at 53 lbs will be charged the first pet rate for 26-55 weight class.

Pet 3 at 63 lbs will be charged the first pet rate for 56-80 weight class.

Pet 4 at 68 lbs will be charged the 2nd pet rate for 56-80 weight class.

Pet 5 at 72 lbs will be charged the 3rd pet rate for 56-80 weight class.

Please note:  If a pet has a default board rate, KC will charge that instead of the weight class price.  The next pet in the same weight class will be charged the 2nd pet rate.