How do we give an app away for review?

Hi all,
I searched the forums but couldn't find this discussion. Apologies if I'm re-hashing an old post.
We have been developing an app and we have it in google play market. We charge for it, but also want to be able to give it to reviewers and contributors.
Google play market doesn't appear to have the ability to allow a voucher holder to download the app for free, and we don't want to risk simply giving out the apk file.
How do others manage giving out free-bees?

Thanks