The easiest way to join my team is to bring your own funding. That’s how I funded my own stays both at the PhD and postdoc levels. You wouldn’t believe how many of the people writing to me have not even thought about looking into this option. Many countries offer grants to pay for PhD and postdoc stays abroad!.
The second best way is to grab my attention. For postdocs this is “easy”: we are probably from the same or similar fields (otherwise why wouldn’t you want to come work with me? it’s just because I now live in Barcelona?) so I should know your work or, at least, be able to read it and get a good idea of your skills. For prospective master / PhD students, this is trickier.
So, how do you grab my attention if I don’t know you already? The solution is easier than it seems: contribute to one of the many open-source tools of the team.
If you don’t find any to contribute to it probably means that none of the topics interests you or that you don’t feel comfortable programming. Any of the two disqualifies you from joining the team. In that case, don’t waste your time and mine!