[color=#1C2837][size=2]Let's say I'm working with three friends on a Unity game, and we need a Pro account.
Would we need to pay the license once, or four times? one per person using unity?
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[color=#1C2837][size=2]And another question:
Would starting an official company change that?
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And let's say someday I start another group, with other people, would we need to repurchase a license, or could we use my license, that I used for this earlier project?
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[color=#1C2837][size=2]ps. We live in the Netherlands.

Why not ask the unity team directly?

According to the EULA: [color=#444444][font=Helvetica, Arial, sans-serif][size=2](d) A company, educational institution, incorporated entity, or individual may not license both Unity Pro and Unity at the same time.[/font]

So I think you're ok as long as you're separate individuals, and only one individual actually uses the pro license (say to put everything together). If you become a company, you'll all need to have the pro license.