Why not fork from the latest version? It's provided as AGPL, and you don't need to sign the CLA as you are not going to contribute to the upstream anyway.
Because they haven't done anything terribly interesting anyways, so unless I prefer AGPL there's no point. I'm pretty sure I will wind up diverging very quickly if I actually commit to this.
And upstream doesn't have significant intentions for dendrite right now... Relevant btw re: licensing: