we want to find a cheap way to travel. originally we wanted to take the night train, but saw it is very expensive.
Well, it depends what to compare it with. In case you decide to travel with the
state railways companies, then minimum price will be 19 euro (ticket + 6 berth couchette reservation) per passenger, thus making it around 100 euro for all five (in case you prefer sleepers, then it will be min. 25 euro per person). However great thing about Czech-Slovakia connection is that it's not monopolized by state rail companies, thus you can travel also with private one. E.g.
RegioJet which offers tickets for the overnight Prague to Poprad for as low as 10 euro (seats), 13,60 euro (couchettes) or 23,60 euro (sleepers). What it means is that all five can travel for as low as 61,20 (5 berths in 6 berth couchette) or 106,20 (3 bed sleepers).
Overnight vs. day time dilemma: during daytime cheapest discounted ticket you can get will cost you 66 euro (1795 CZK) with state railways or already mentioned 40,50 euro with RegioJet
and back with a stop on the way back for 2 days in the area of mala fatra (zilinia)l
You can purchase same discounted tickets from Zilina to Prague as well, it will be even cheaper then Prague-Poprad with state railways. The only thing you should take into consideration that Zilina to Prague it's just 5-max.6 hours journey with overnight trains, thus it might not be very comfortable night (e.g. boarding the train at half past midnight and getting off at 6am)
on leg Poprad-Zilina (or maybe some earlier station like Ruzomberok?) you can just go with Slovak domestic tickets or perhaps even rely on buses (depends on where exactly in Little Fatra you need)
one of us might come back separately before the others.
Not a problem, just purchase a separate ticket and that's all