Well, this is actually pretty simple. You need 3/3 in Blood Worms in the blood tree.
WorldofWarcraft.com -> Info -> Classes -> Death Knight -> Talent Calculator
To get the bloodworms, you need to use Death Strike.
Once you get the blood worms, and kill somebody, cast "Raise Dead" and your Bloodworms worms will also turn into ghouls and you will have around 8 ghouls following you. However, if you go somewhere where there is a bunch of people, and you get Bloodworms there, and you cast "Raise Dead", then you will get a ghoul from all the worms that you got, then from the dead corpse, and also everybody who is around you. I did this in thrallmar and got about 50 ghouls to follow me.
Tested and Working as of August 12 on official WotLK Beta Realms.
--UPDATE--
Go to Spinebreaker Post and cast Raise Dead there, you will get like 40 ghouls.