The new macOS High Sierra 10.13.5 beta can be downloaded through the Software Update mechanism in the Mac App Store with the proper profile installed.
It's not yet clear what improvements the fifth major update to macOS High Sierra will bring, but it's likely to include bug fixes and performance improvements for issues that weren't addressed in macOS High Sierra 10.13.4.
We are waiting on several features to come to macOS and iOS, including Messages on iCloud, a feature that was in the iOS 11.3 beta but later pulled. Messages on iCloud has been reintroduced in the iOS 11.4 beta and is likely in macOS High Sierra 10.13.5 as well.
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.
We'll update this post should any new features be discovered in macOS High Sierra 10.13.5.
Update: The macOS High Sierra 10.13.5 beta is now listed in the Apple Developer Center.