Create an place like an arena. One entrance which can be only open by a special key. This arena might include some grandstand where everyone else (without a key) can enter to be become a member of the audience.
The GM can send invitations to players which are special in some way. This invitations are "ordinary" items. A letter with e.g. the following text:
The invitation-letter acts as a temporary key to the arena door. However, it is time limited to one event. Players can not enter early or later then the aboved mention time.
At the specified time the game master will enter the arena (maybe there is a small tower in the middle of the raid field, so the GM can stay safety). He will open the raid and create some monsters. (Maybe the kind and amount of monsters will be announced or player can ask during the raid)
All invited players can fight. No special rules, if they die they die, with all consequences.
After the raid is over everyone has to leave the arena.
Ready for the next round.....

The invitation can be send by different criteria. E.g.

The raids should be something like a personal award for the players.

Players which are not able to participate can give there invitation to other players if they like.

In addition to that it might be easy to generate some additional quests-raids.

E.g. Two groups, and a huge amount of Elfs or Orks. The creatures protect an item. The group which is able to catch the item earn XP or cash.
Protect the civilest: A amount of players and one pseudo-player level 0 stay together, a bunch of monsters try to kill the low-level player. If the others are able to protect him, they win XP or cash.
Raids were potions and meat are forbidden will be definitive a hard task even for high level players, where team-play becomes the most important criteria.

That is more or less all.

StendhalRefactoring

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