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You should find them under a small heading of "Archaon's Horde Army Selection", it details the upgrading of command groups there.
As for the daemonic command, there is none. The only unit that actually gets a champion is the Bloodletters, so any others may not have any command models.
Oh in the DL list, yes the core units may take command groups. I thought you meant in the HoC book, but in the DL list it says under the core unit entry as to what may take them and the costs.