The Arduino team collaborates with a lot of people and we accept contributions from everybody.
In any open source project there is a "benevolent dictator" that has to make decisions in the end following the original spirit of that made the project successful in the first place. (you can find that concept in a lot of writings about Open Source)
Having said this we're not many people and we can't cover all areas as quickly as some of you would like.
The criteria is very simple, I'm looking at the ones who have reported the most posts while considering the amount and quality of their contributions. I've started from the "non-english" forums and now I'm working on the international forums.
There is still a lot of work to do.
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