Hi @Julian Brunner,
Thank you for posting in this community.
Depending on your needs, I don't think you need to configure additional licenses for your guests.
You just need to set corresponding permissions for your guests so they can access the sites and upload or download files.
Below is what the official Microsoft documentation states that guests without a license can do.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.