Hi erwincoumans! Yes, this is absolutely possible. You are able to create your own ‘Organization’ on GitHub and add fellow subscribers to your organization’s team, including granting them write access to your organization’s private fork of Unreal Engine. The team members must all be verified subscribers, otherwise this is a violation of the license agreement.
If you fork the UnrealEngine repo via GitHub only subscribers will have access to it. You can easily verify this by logging out of your GitHub account and then attempting to access your fork’s GitHub page, you should get a 404.