A people deck.
Emissaries aren't exactly deck building. Basically they're just a simple way to expand on the number of combinations of decks (and use the new cards) since you can combine each people deck with one of the four Emissaries.
The exact rules are on page 5 of the Advanced Concepts rules.
Players looking for a quick deck-building experience can use emissary cards. Both players must agree to use emissaries before playing a game. When building a deck with emissary cards, players perform the following during step 3 of setup:
Add People Cards: Each player adds the three cards without a deck indicator of his chosen people to his deck.
Choose an Emissary: Each player chooses an emissary card and places it in his leader area, making sure both his leader card and emissary card are visible (see “Covering Cards Diagram” on page 11 of the rulebook). Then, he adds all cards with his emissary’s set indicator to his deck.
It's a bit confusing because this section comes right after the section on deckbuilding and moon values and theoretically you could build a legal 30 card deck then add an emissary to it, but this is not how the emissaries were originally intended to be used.
From dearlove's excellent FAQ at http://www.mnemosyne.demon.co.uk/bgames/bluemoon/
Q: How do the emissaries work?
A: The emissaries provide a fast play mechanism. With both Emissaries and Inquisitors decks available, you each start with a preassembled Blue Moon people deck, then add the three cards for that people from the Emissaries and Inquisitors decks. You then select an emissary,which lists seven cards from its deck which are not from any of the Blue Moon peoples. This gives you a 40 card deck (the emissary goes with your leader). With only one Emissaries and Inquisitors deck, four peoples produce 39 card decks and four peoples produce 38 card decks, because the eight Chosen Ones are divided four in each Emissaries and Inquisitors deck. Note that when all peoples and emissaries have been played, you will have used all of the cards in the Emissaries and Inquisitors decks other than the inquisitors.(Each Emissaries and Inquisitors deck has 2 emissaries, 2 inquisitors, 12 Blue Moon people cards and 14 cards divided among the two emissaries, making 30 cards in all.) The emissaries are not used with the Advanced Rules.