The new macOS High Sierra 10.13.5 beta can be downloaded through Apple Developer Center or the Software Update mechanism in the Mac App Store with the proper profile installed.
macOS High Sierra 10.13.5 introduces support for Messages on iCloud, a feature that was previously present in macOS High Sierra 10.13.4 betas before being pulled ahead of the release of the update. Messages on iCloud is also available in iOS 11.4.
The update also likely includes bug fixes and performance improvements for issues that weren't addressed in macOS High Sierra 10.13.4, but as Apple does not provide detailed release notes for macOS High Sierra updates, we may not know exactly what's included until the new software is provided to the public.
No major outward-facing changes were found in the first beta of macOS High Sierra 10.13.5, but we'll update this post should any new features be found in the second.
The previous macOS High Sierra 10.13.4 update brought support for external graphics processors (eGPUs) along with Business Chat in Messages and several other bug fixes and smaller feature improvements.