External User - Collaboration Licenses

We are currently on a project where we are sharing our takeoff with a 3rd Party Cost Estimating Team in an effort to reduce takeoff hours and costs for the Owner. The Owner loves this idea, but getting new softwares approved and paid for is always an issue. Our team would like to be able to find a way to be able to selectively give access to External User in a "collaboration" capacity to be able to review and make comments on our takeoffs without requiring them to purchase their own separate licenses. Typically this would be a group of 3-5 external users for a finite period of time, like 2-3 months at a time.