AEM Communities creates a special case where new content is expected to be created on AEM publisher servers. Users can create content or users can be added or removed. For best performance use the TarMK for publish servers. But one thing that is problematic is that TarMK is not clustered and information would have to be reverse replicated to the author, and then replicate to each of the publish server. That way of syncing publish servers is slow and involves many more steps.
Within AEM 6.0, when users create quite a bit of content with AEM Communities it is suggested to go to a clustered configuration. This negates the performance advantages of TarMK.
AEM 6.1 introduces a new method to sync between publish servers, a tunneling service between publishers. Sothea Nim of 6D Labs provides an example of how to take advantage of this new functionality. 6D Labs | AEM6.1 – Sync Users/Groups on Publishers