PowerBI License for Guest Accounts

What is the proper procedure for the following scenario.   We are contracting through a 3rd party partner to author PowerBI reports for our company.   Ultimately, our internal employees will use these dashboards and reports.   Until then, this partner will need access to our data to create these reports.   With this said, do we need to add them to our organization's AD/tenant?   Do we need to purchase a PowerBI license for them (we are on E1 and E3; and currently purchasing individual Pro licenses).   

I've read a little bit already about this topic from a previous posting from a year ago.    I saw there is an option to add an external account to an organization's tenant without purchasing a new AD license.   If this is the case, can they bring their own PowerBI license or do we need to supply them with one?   

Thanks.