FAQ : Can I use Sharepoint Online for my Extranet?

  • Article

Yes you can, and you do not need extra licences for the first 50 external users (as that is included in both Sharepoint Plan1 and Plan2).  If you need more than 50 external users you can purchase additional external user licence packs in blocks of 50. 

For more information check out this link https://community.office365.com/en-us/f/152/t/6880.aspx