Bigger is Better

This quest chain opens your level 2 garrison.
This can all be completed at level 90 (but you will probably hit 91 along the chain from quest/kill xp).

Once you have the level 1 garrison, the quest chain continues at:
A Hero's Welcome
=> Closing the Door + Dark Enemies + Think of the Children!
=> Shadows Awaken
=> Into Twilight
=> The Clarity Elixir
=> The Fate of Karabor (Solo Scenario)
=> Going Undercover
=> Escape from Shaz'gul
=> Fast Expansion (sends you back to Garrison)
=> Bigger is Better (need 200 Garrison resources to upgrade)

As of the Shadowlands pre.patch (9.0.1), there are two ways to get this quest, which depends on what level your character is:
Level 10-19: You can get this quest after completing the quest chain that starts with A Hero's Welcome, which you pick up from Yrel inside your garrison.
Level 20+: You no longer need to do the quest chain from Yrel and you get this quest as soon as you complete Delegating on Draenor (the quest to start your first Garrison Mission)

It still appears to be bugged on launch day.

I found to fix it, just leave the garrison and return (I left it a little bit, did a few quests and then came back). When you come back, he should be there in your town hall.

Oh you also don't have to be level 92 to do this quest as I was 91 at the time.