We Don't Know
The entire Dresden Files series is rather vague on this point. Certain events within the series also make it unlikely to come up. Each of the Vampire Courts seemingly represents some subset of the modern interpretations of vampires. The existence of the more traditional Black Court and the hereditary White Court, as well as the use of the term "Infected", suggests that the transformation may be viral. However, the official answer from Our World is
whether this is by a massive overdose of narcotic venom, a different sort of venom, or something else is unknown
This is quite a hard question, so I will do my best to summarize what I've found on Orcus and Demogorgon. Also, the 4E Book of Vile Darkness has a lot of information about demons lords.
Orcus
- Lieutenants: Vampires, liches, necromancers, cult priests
- Minions: Undead (ghouls, shadows), cultists
- Races: Not relevant for undeads, humans, orcs, half-orcs, ogres and giants
This may be the easiest one, because of all the detailed description about him. From the MM p. 52:
Orcus is worshiped by the undead and by living creatures that channel the power of undeath.
Which makes the choice for a lieutenant relatively easy. On multiple wikis (like here or here), liches and vampires are mentioned as his most powerful worshipers. In the 4E published adventure Keep on the Shadowfell, a human priest of the Cult of Orcus tries to open a portal to the Shadowfell. Also, from the MM p. 53:
Orcus, the Prince of Undeath, has the power to transform manes into undead monsters, most often ghouls and shadows.
We can assume he does that to make them serve him as minions. From the Book of Vile Darkness p. 138:
In particular, orcs, half-orcs, ogres and giants revere Orcus, as do a large number of corrupt and despicable humans.
Another quote from the BoVD p. 138:
Intelligent undead never willingly serve Orcus (they are more likely to venerate the deities Vecna or Erythnul). However, many vampires, liches, and other undead creatures are forced into his service by dark pacts or compelling magic.
Demogorgon
- Lieutenants: Severik the balor (mentioned in the BoVD p. 129)
- Minions: hezrous, aboleths, scrags, skum, various ichytoid beasts (BoVD p. 125)
- Races: tiefling, lizardfolk, half-elf (examples from the BoVD), kuo-toa (Out of the Abyss), Derro (Out of the Abyss). Mostly any kind of creature that has reached a pretty high level of madness.
Not many quotes for the Prince of Demons, but you will find more details in the BoVD.
Other demon lords
Well, when I started writing this answer, I thought I could gather information from websites, books and the like, and summarize my findings here. It turns out, the Book of Vile Darkness from 4E is really the best source of information about the demon lords, so you should probably buy it for the other ones.
Best Answer
Demogorgon “is capable of devising and enacting the most clever strategies” and the mere sight of him compels demons to obey his command.
Demogorgon has 20 intelligence. 20 is as high as an ability score can get for player characters without very powerful magical items. So think of the smartest wizards in all the land. Demogorgon is about that smart.
Demogorgon is described in Mordenkainen’s Tome of Foes:
So yes, Demogorgon is very strategic. But can he command demons?
The mere sight of Demogorgon commands obedience from lesser demons throughout the Abyss.
As for the rest of the demon lords, the dumbest of the bunch is Yeenoghu at a respectable 16 INT1, with the most intelligent being Fraz-Urb’luu at 26 INT.
1 The printed edition of Mordenkainen’s Tome of Foes lists Yeenoghu’s INT as 15, but lists the INT modifier as +3.