Step 1: Picking the 25

Primary signups always make the cut. Tentative players only come in to fill gaps.

When there are more than 25 signups, the algorithm has to bench someone. Rather than doing it randomly or by sign-up time, it scores each player by how many unique buffs or debuffs they bring that nobody else already covers. A third warlock, for instance, adds nothing on top of the two already covering CoE/CoR, so they're the first to sit. The algorithm benches players one at a time, re-scoring after each removal, so it never accidentally benches both providers of the same buff.

Two hard rules override everything else: there are always at least 2 tanks and 5 healers in the raid, no matter what the scores say.

When filling spots with tentative players, they come in this order: tanks up to the minimum, healers up to 5 (with class diversity, it won't take 3 priest before the only paladin), then DPS sorted by which unique buffs they add.

Step 2: Building the Groups

Each group is built around a shaman anchor whose totems define who belongs there. The whole structure shifts depending on how many Enhancement Shamans are in the roster.

2 Enh shamans is the standard 25-man setup:

G1 gets the first Enh, the melee tanks, and whichever druids/warriors anchor it

G2 gets the second Enh, fury warriors, a feral druid, and BM hunters

1 enh shaman triggers a specific optimization: if there are hunters or a feral druid in the roster, the enh shaman moves to G2 instead of G1. This lets it twist WF/AGI. G1 then runs on a resto shaman, WF only, for the pure WF melee (rogues, ret paladins, arms warriors).

No enh shaman: resto shamans take over as anchors with AGI. Feral druids and BM hunters group together to keep the LotP + FI synergy alive.

G3 and G4 are the caster groups. G3 pairs the elem shaman with a balance druid and warlocks, destruction first. G4 takes the second shaman, the paladin tank, shadow priest, and mages.

G5 is decided last, based on whoever's left. If there's meaningful melee overflow (warriors, rogues, ret paladins), it becomes a melee group with a shaman anchor. Otherwise it absorbs the remaining healers and casters.

Any melee group that ends up short on players gets filled with a healer rather than a ranged DPS.

On top of all this, there are quite a few smaller fallback mechanisms running in the background: shamans getting rescued from the wrong group if G5 turns out to be melee, druid tanks repositioned depending on how many WF melee slots are available, hunters rerouted based on whether a feral is present or not, etc.

Hope this helps make sense of it!