Hi Stefan,
According to your description, you can share the team site with external users.
If so, you don’t need to purchase the additional licenses.
To do this, you can refer to the article below:
http://office.microsoft.com/en-001/office365-sharepoint-online-small-business-help/share-a-site-with-external-users-HA101850586.aspx
Regards,
Wilfred Ying
