Please provide a way to turn on Delve/Office Graph for only a subset of users!

I agree this would be a sensible option to have, where organizations want to dip their toes into the water with Delve.  I am not sure Microsoft have a motivation to provide this, though.

We looked at disabling Office Graph when management thought Delve was revealing information it shouldn't be, which was more a misunderstanding on how the technology works.  

We dealt with this via training being upfront and letting staff know what Delve does and doesn't do.  This was the discussion I posted at the time - Disabling Delve?.  Afterwards, jcgonzalezmartin posted a useful addendum - What functionality could I possibly lose if Delve is disable in my tenant? which illustrates how much is lost when not enabling Office Graph.

Anyway, I think you might have to spin up another tenant in the meantime and hydrate it was some users and content to look at Delve as a PoC etc.