Where is the NPC for doing proving grounds in the Dreamgrove?
Solution 1:
Basically every class trainer + the guy at the mission table in the WoD garrison sends you there (and the panda guy from MoP at the Temple of the White Tiger).
In Legion you'll find class trainers in some class halls. But the Dreamgrove is one of them without a trainer, so there is no way to get to the Proving Grounds directly from there. Shamans for example would have one.
Solution 2:
In Legion you will find class trainers by the portals in Dalaran (Broken Shores). Most classes are there, but not mages, DKs, or Demon Hunters.