@Kathrin Swoboda - You can invite an unlimited number of registered Miro users to work on a board in your Free Plan team, however, they must be a member of your Free Plan team.

NOTE: This means that they will be able to see all boards in your team, and create new ones. If you do use the Free Plan team for this session, you will want to delete the users from your team one your session is over.

You can generate a link that, when sent to someone, they will be automatically added to your team as long as the Enable team invite link option is enabled:
Go to Team profile → Permissions → Invitation settings:

Then open the Share settings of the board that you want them to work on with you and use the Copy invite link button:

When the session is over, you can delete them from the team: