Does anyone have experience squeezing 5 skiers into a hire car?

You need a roof rack (Dachgepäckständer, whatever this may be in English).
Then you can transport your skis on the roof. However, you might have a
problem to cram all the luggage into the boot, and you do need winter
clothing, sticks, boots. A Ford Fiesta might be somewhat small.

Good luck!

Turan