I read that a Major Rewrite was in progress for Groupjive.
Not yet it's not. Currently working on CB Activity. Once it is stable I'll begin the GJ upgrade. Once the upgrade is released I can begin the rewrite.
When will this be ready. Will the current Groupjive version 2.7 be compatible with the new version.
GJ 2.8 is just an upgrade not a rewrite. The rewrite won't be until after the upgrade and migration will be done automatically on install.
Should I wait?
No, the rewrite won't be for some time. The upgrade should be started within a month or so depending on CB Activity progress.