Hmm, it might be a bit late in the game to be doing it now. Often with the more serious-minded organisations, you will need a person of some degree of seniority or influence to vouch for you before you will be welcomed into their activities, as a person who is traveling internationally.
Often the more seasoned organisers will have contacts with international organisations and you can get kind of circle of trust thing where, for example, your local organiser can vouch for you to your party leadership, your party leadership can vouch for you to their contact somewhere like in Greece, and that Greek contact can connect you with whoever their contact is in Amsterdam.
If you're already there then I'd look for radical bookstores or cafes/bars that are radical. Often it'll be an antifascist cafe or bar that you'll need to hunt for.