Agree that Ryanair really pester you about their bag policy, I can't understand how you can miss it. It's mentioned several times during booking, they email you regularly between booking and flying, when they start to pester you about booking seats, because you can pay extra for that too and then during the check in process.
You say it was a group booking - did someone else book the flights? Then your argument is with them not not passing on a clearly communicated message.
We've also found Ryanair to be usually significantly cheaper than Jet2 or Easyjet. We used to fly jet2 a lot, but they seem really expensive recently, and as for BA, forget it, they provide hardly any flights to European holiday destinations from northern England, so we don't really have the choice of using them anyway.
Surely it's common knowledge that all airlines vary slightly in the rules, charge extra for lots of things (baggage, seats, onboard refreshments) so you have to check exactly what's on offer, not blithley ignore all the information, and then moan when you get caught out.
Of course it would be a lot easier if the price did include all the extras and there wasn't lots of tricks to be aware of, but then you wouldn't get the bargain basement flights, because there simply isn't any profit in a £30/50 return flight as £26 of that is tax anyway.