Darth Bane, the creator of the Rule of Two (one master and one apprentice, no more, no less) described the non-Sith Dark Jedi in Darth Bane: Path of Destruction:
Minions and servants could be drawn into the service of the dark side by the temptation of power. They could be given small tastes of what it offered, as an owner might share morsels from the table with his faithful curs. In the end, however, there could be only one true Sith Master. And to serve this Master, there could only be one true apprentice.
Bane had a network of spies, but doesn't seem to have trained any of them in the ways of the Force. Perhaps he felt that it was better to do without training force-sensitive minions to prove that his Rule of Two worked.
However, Palpatine most certainly used force-senstive servants.:
- Inquisitorius: Trained in lightsaber combat and tasked with hunting down and killing Jedi who survived Order 66. (This one is the only one on the list that is in Disney continuity)
- Emperor's Hand: Training in infiltration, deception, lightsaber combat, and long-distance communication (Mara Jade was a notable member of this)
- Emperor's Reach: Able to travel vast distances using the Force
- Emperor's Eyes: Devoted to seeing into the Force and the future
- Emperor's Voice: Another group, but it's not clear what their training was
As you can see, each group was trained in specific skills, rather than being taught all of them, which Palpatine knew. The idea was that all of his knowledge would be passed to his apprentice, but each servant would only have part of their knowledge.
There's also the interesting case of the Prophets of the Dark Side. Darth Millennial was the first Sith to reject the Rule of Two and he fled as an apostate to found that group. But by the time of Palpatine, they were no longer persecuted and were in fact used in the same way his other groups were. Again, Palaptine didn't try to train them in everything he knew.