How do you open all 4 doors at once?

One person for each door?

One person opening two doors on one side and another person doing same on other side?

Ok seriously

Does pressing unlock twice do it?

Or do you mean when in the car?

To let several persons into the car. Didn't get a manual. Press button twice? Great, thanks.

Try the press button twice, that's a guess from me.
If doesn't work come back here and say.
Also get a handbook , well worth it lots of tips and info how to look after your car

what kind of car do you own ? At least here in Austria, there is no 500L without center locking.

If it is the 500L you own, there's a few manuals scattered about online if you're not able to get hold of one.

https://www.fiatusa.com/download/pdf/manuals/2014-FIAT-500L-OM-1st.pdf