@Rune Gamborg You can't upgrade to Enterprise until you have > 20 accounts.
At that point, you gain control of users/boards/teams and also users by domain, however you can still add any other member to any Trello board. The don't have to be on your teams, in your domain etc. The boards operate the same, but you get more control over them so for example when someone leaves after having created a bunch of boards you can control those boards.
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