Group Chats: Who Can Put You in One, and What Leaving Actually Does
Until today this site had exactly one sentence about group chats: the blocking article saying a group is not a way around a block. Everything else about rooms with more than two people in them was unwritten, and reading the server for the answers turned up two surprises, one of which was a sentence on the messages page that was wrong and is now fixed. The answers are short.
How You End Up in a Room
Two kinds of conversation can hold more than two people, and they arrive in your list by completely different routes.
Here is the first surprise. No page creates a named group. The messages page can start a direct message with anyone by username; it has no control that starts a group, and neither does any other page in the game. The rule above is what the server would enforce if one did. So today, the only room you can be put into without being asked is a guild's, and you put yourself there by joining the guild.
The one thing that holds for both kinds
A room is never a way around a block. The server refuses to create a group with anyone who has a block between them and the creator, and the blocking article covers what a block does inside a room you were both already in. None of that is restated here.
What Leaving Actually Does
For a named group, the button in the conversation list reads Leave group, and this is exactly what it does, in order:
That last row is the second surprise, and it is the sentence that was wrong. Until today the confirmation you tapped through said the room and its messages would stay for everyone else and a member can add you back later. No member can, because nothing can. The confirmation now says so: the room and its messages stay for everyone else, and nobody can add you back to it.
A guild room has no leave button
You cannot leave a guild's room on its own, and as of today the messages page does not offer to. It used to show the same Leave group button on guild rooms, where the server refuses it, so the button could only ever fail. A guild room is yours for as long as you are in the guild; leaving the guild is what removes you from its room.
What the Room Cannot See
Muting, pinning and archiving a room are yours alone: the room does not know you did any of them, and neither does anyone in it. Yesterday's spotlight has the full table of which controls the other side can see, so it is one link here rather than a repeat. One thing worth adding because rooms make people wonder: being in a room with someone does not make you friends, and presence is friends-only, so a room does not tell anyone whether you are online.
Do not think of a room as private
Nothing about a group is hidden from its members. Everyone in the room sees everything sent to it, including the line that says you left, and what you send is what stays: a report always has the message it refers to. If a room has become somewhere you do not want to be, leaving is a complete exit and it is the exit the game gives you. What it is not is a way to unsend anything.
The Short Version
For the guild side of all this, how a guild is joined, run and left, the Guild Hall guide is the place, and it covers the room only as one of the things a guild comes with.