you can't

you can't because you cannot friend

i know the answer, i looked inside a code and found a solution.
The game changes password immedialtly when the game begins, its random, so.
your friend shall join you only before game loads, when youre on the loading screen.
Warning: 1.Selected playermodels wont appear, instead your friend and you will look like gray gordon freeman(aka default hl2 playermodel)
2.Hunter will only chase one player.
3. Sometimes there will be bugs with items.