In the gamemember

Raids › Raiders

Who is at the level cap, which roles they are willing to play and whether their gear is ready.

In your game: Raids › Raiders

What it does: Lists the guild members at level 60, online or not, with their roles (T, H, D), gear state (Ready, Gearing, Missing), iLvl, spec and a note. At the top, cards count raiders, online, tanks, healers, dps and ready.
Who sees it: Everyone sees it. Only officers change roles, gear and note; for the others the list is read-only. Everyone reads the note an officer writes here.
Where to find it: In the window (J or /gos), Raids tab, Raiders sub-tab. You set your own roles in Settings › General › My Profile.

Every member at level 60 is included, offline ones and alts too, in alphabetical order. There is no way to add or remove someone by hand.
Each person's roles are the ones they ticked in My Profile, until an officer touches the row. From then on the officer's roles apply, starting from the ones the person had ticked.
Gear cycles with each click: —, Ready, Gearing, Missing.
The note has up to 120 characters.
Whoever has more than one role counts in each of them in the cards at the top. Ready counts whoever has Ready gear.
An officer's change reaches the guild in seconds, and whoever logs in later receives the list from an online officer.

iLvl and Spec showing —: the person does not have the addon, or their addon has not sent that data yet.
Changed your roles in My Profile and nothing changed here? An officer has already touched your row, and their choice applies.
The roles here (T, H, D) are not the cores' roles (Tank, Healer, Melee, Ranged). They are separate lists.