
Recent Allies remembers every player from a group
World of Warcraft: Forever keeps a list of every player a character has grouped with. The Recent Allies pane sits next to the Friends List and saves name, level, class, place and activity. Wowhead showed it from the beta.
What the list saves
Every time a player groups with someone, a record lands in Recent Allies, writes Wowhead. It holds the name, the current level, the class, the place where the two met and what they did together. Players who are online get a button that invites them in one click.

In the screenshots from the beta, every entry reads Fought Together. The tooltip also shows the race, the faction and how long ago the activity was.
A recent ally can be pinned, so the name always stays at the top of the list. Whoever plays with the same person often enough can still add them to the friends list.

How many players the list holds and how long an entry stays, Wowhead does not say.
Part of a social design
Forever leans on playing together, and Blizzard has said in several interviews that friction helps. In an interview with Game Informer, two leads called running to a dungeon or a long flight path a moment in which players talk to each other. In an interview with Windows Central the team said the same the other way round: remove the friction and the game gets lonelier, writes Wowhead.
The missing shared tagging fits that picture. Only the first player or group to hit a mob gets the kill, and that pushes players into groups. Wowhead sees Recent Allies as the next step: whoever groups up for a camped mob or a dungeon ends up with a list of names to look up later.
Source: Wowhead