Frankly I want to be able to join the shadow druids and help them complete their takeover and throw out the filthy untouchables. Shadow Druids must have secrets and power I can exploit.
As it turns out thought they won't let you join up. You can only Kill or Redeem Kharga.
They indeed dont let you join up ...
Even tho i would dare to say it makes pefect sense, Shadow Druids (and feel free to corect me) seems to me like a group that see themselves as quite elite club ... so they would hardly accept any adventurer, who would just say "hey sounds good, let me in".

Anyway ... you totally can help Kagha and Shadow Druids to take over the Grove ...
You just dont get any reward for that.
//Edit:
What would work well (for me at least) would be that if you help Shadow Druids ... and you then tell Kagha that you know about her little betrayal (wich you can) ...
One morning you would find some pouch next to your bed with few gold pieces and some mystery note like "we will be watching you". :3