How can I add team members to my board who have already their own miro account?

In our working structure, I often have to collaborate with other individuals and free lancers in different projects. I have my miro consultant plan, they have their own miro consultant plan. So everyone has a paid account. Now I try to invite them to jointly work on one project or board, but it seems, that we would have to buy an additional team member seat, which then would multiply our cost. Did I do something wrong or is there really no possibility for flexible cooperation in projects as I know it from other tools?

Thanks for any suggestions,

Patrick