UPDATED: Join Together v7.8.0 and M3Unify v1.9.1
I've just released new versions of Join Together (v7.8.0) and M3Unify (v1.9.1) that make some initial accommodations for macOS 10.13 High Sierra.
To date, there have been a few issues with the High Sierra betas, as one expects. But the latest versions of these apps should be fine running in the latest betas. There may be some additional updates when the final version of macOS 10.13 is released later this year.
Even if you're not running a High Sierra beta, you should update to these new versions of Join Together and M3Unify since they also have performance fixes and enhancements. And everybody likes those.