The individuals you share files with outside of your organization are seldom the only ones who require access in order to meet the business need for sharing the file in the first place.
When the need to re-share files arises, you and the outside party have options, but all come with significant drawbacks.
The recipient of a shared file could download the file (assuming you allow this) and pass this onto the other party. But if online editing and co-authoring of the document was your intent, the other party can't participate.
The recipient could reach out to you and ask you to send the file to the other party or add them to the list of persons authorized to access the shared file, but this is a hassle for both of you and will take some time to coordinate.
But why not allow the recipient of a shared file to assign their rights to access the file to a 3rd party?
With e-Share this can be easily done, either on a policy-basis or a case-by-case basis. And you can optionally require the data owner's approval for this re-sharing. Better still, you can require this approval:
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