I would vote to close this question. The close reason is: "Questions that ask for recommendations of games, servers, software, or hardware are off topic" which is exactly what this question is asking.
You can't ask something along the lines of "What are the services I can use for XXX", we simply don't allow it.
What you CAN ask is something along the lines to this question: How should I go about finding a guild in WoW?
This question doesn't ask for a list, but instead asks for (official or unofficial) ways to do what the previous question wanted.
Official places are not some kind of recommendation and guilds are a very important and major part of the game. In fact you can't beat the endgame content without one. It just doesn't work alone.
Asking for a list of official places is a recommendation. If I asked the question "What in the Steam Workshop allows me to add more dungeons in Skyrim?" that is off topic. Just because it uses an "official place", is still a recommendation. You can alter your question to "Is there a way to add additional dungeons to Skyrim?" and the answer may lead you to the Steam Workshop.
The fact that guilds (or anything that any question asks) is important to playing the game is irrelevant to the rules. No matter how important any aspect of a game is, should have no merit to the legitimacy of the question.