How do I pass an array as function argument

I think you need function with "by reference" variable. By default all function parameters in MaxScript are ByVal (i.e. pass a copy of the value), not ByRef (we marked explicit with '&' sign). And for example:

myArr = #(1,2,3) --> our initial array
num = myArr.count + 1 --> 4
append myArr num --> will modify our myArr to:
#(1,2,3,4)

But... using function:

myArr = #(1,2,3)
fn myFunc arr = (append arr (arr.count+1))
myFunc myArr --> will return NEW array = #(1,2,3,4)
-- but will NOT modify your input myArr

So, you still can assign the result of your function to your initial array like this:

myArr = myFunc myArr

Or if you want start using ByRef, then here is an example:

myArr = #(1,2,3)
-- now take a focus on the param definision (&refArr):
fn myFunc &refArr = (append refArr (refArr.count+1))
-- and take a focus on how we call that function:
myFunc &myArr -- i.e. again using '&' followed by your array name
-- now your myArr is directly modified.

Ah, and as all functions always return result, using ByRef you can combine to make your function return multi-results. Here is an illustration --

-- lets say we want to filter FreeCamera and do something with them...
camArray = for c in Cameras where classOf c == FreeCamera collect c
if camArray.count > 0 do (
	for c in camArray do ...
)
-- but we intend to wrap this pre-setup in function...
fn filtFreeCams &outArr = (
	-- we modify/assign result to variable out of the function:
	outArr = for c in Cameras where classOf c == FreeCamera collect c
	-- and supply boolean out result for our function:
	outArr.count > 0 -- i.e. False if its empty, otherwise True
)

-- so, now we can write our code like:
camArray = #()
if (filtFreeCams &camArray) do (for c in camArray... )

Hope this helps, Cheers!

thx miauu, yeah I tried that too but didn't work somehow. anyway I did it another way now, but thanks anyways

(
fn myfunc arr =
(
--do some stuff with array
append arr 3
)
arr = #()
-- myarray=#(a,b,c,d,e) -- #(undefined, undefined, undefined, undefined, 2.71828)
myfunc arr
)