So how does the event actually work?

In the first round, teams of 3 to 5 players will be formed. The primary goal when choosing these teams is that each individual game is fairly well balanced. There is also a minor weighting towards teams with similar skill variance.

In subsequent rounds, new teams are assigned. These teams are chosen so that in each individual game each team has players with the same (wins-minus-losses) score, or as close to this as possible. Subject to this constraint, we again try to find teams which are fairly well balanced. Minor weightings are applied to promote similar skill variance between teams, and to discouraging players from being on the same or opposing teams more than once. However, in the service of producing balanced teams these minor weightings may be ignored.

What rating is the balancing based on?

Nominally, your "true" casual rating on the day. Actually, it is just whatever number I choose to put in a config file. So don't get any clever ideas about tanking your rating before the event.

What happens if somebody has to drop out? Or wants to join mid-event?

My system should be set up so that players can leave, join, or even re-join mid-event without issue. The constraints are that we have enough players to run at least one decent-sized game, and that we have an even number of players. At worst, I can play or not play each round to ensure even teams.

I reserve the right to blacklist people from future events if they don't play the games seriously, or otherwise waste the time of myself and the other participants.

Isn't it easier to just play small teams games in the lobby?

The idea behind this event is that we are more likely to be able to produce well-balanced games by putting 16 players into two 4v4s than by putting 8 players into a single 4v4. The number of possible permutations to choose from goes up by a lot, even taking into account the restrictions based on player score. The last Quadrille seemed to work pretty well.

How is this going to work without a power-of-2 number of players, and with an undefined number of rounds?

We won't necessarily have a single undefeated player at the end. Oh well. The real point of matching people with similar records is that, hypothetically, people who are having a good day and are playing above their rating will be matched against each other.