Tier pieces need a quest chain with group steps
Raid bosses in World of Warcraft: Forever drop Tier Tokens, not the Tier Pieces themselves. A token starts a quest chain that ends with a piece of your class set. Some steps of that chain need a group, says Blizzard in the launch overview video.
What Blizzard says
The overview video lasts 30 minutes, and the last part is about raids. Raids drop tier sets again, with a bonus for wearing several pieces of the same set. About the tokens, the video says:
Vanquish the raid bosses, and you’ll be able to claim rewards that far exceed what you’ve come across previously. Acquire a Tier Token and you’ll open a new adventure for you or any other ally lucky enough to claim their prize. With these rewards, you’ll need to seek out how to start a quest that ends with an adventurer earning a Tier Piece, one of the greatest items one can acquire for their given class. Much like dungeons and raids, you won’t be able to complete these quests completely on your own. So keep in close touch with your fellow adventurers so that you all can achieve new heights of power.
You first have to find out where the quest starts. Like in a dungeon or a raid, you need other players to finish it.
What is not known yet
How many quests the chain has, where it starts and which steps need a group, Blizzard does not say. It is also not known whether the token works the same way for every slot of a set.
The first three raids open on 9 December. After that, a new raid tier follows about every four months.
Source: Blizzard Entertainment