It's a rather tough nut to crack. Problem is that there are no intra-European labour laws, nor legal ways of making a blanket union regarding all the different bases in different countries. Having said that if i were to start something i would definitely reach out to colleagues at RYR and EZY to find out how are they managing in their respective countries (bases). Also contacting local transportation labour unions wouldn't hurt. All these have to be put in motion, legalized, notarized, the full legal monty, etc. before offering individual crewmembers to sign up. Not sure about eastern Europe, but apparently it's even a tougher nut to crack. Local labour code seems somewhat pro-corporate and a bit of anti-staff/worker which doesn't come as a surprise considering most eastern European countries agenda on attracting western manufacturing with lower labour costs, etc. However i would highly recommend researching the history of ALPA. Good book on the subject is Flying The Line by George E. Hopkins and it's also available as an audio book on a free podcast running by the same name. I know, it's US, fundamentally different from the EU but still gives one a good picture on what's involved and how much hard work has to be put in.