@AndreaKrejci - While the Free Plan allows for unlimited members/users, only three boards can be editable at any given time - this limit is at the team level and not per user:

If you wish to have you own team, you can leave your colleagues team--you’ll actually have to leave all teams--at which point Miro will prompt you to create your own free team. Once you have done this, your colleague can invite you back to their team. You can then switch between team dashboards from the left-vertical bar:

To leave a team, go into the Team profile settings:

And then scroll down and Leave team: