Because I’m not positive about Group Loot, I can’t give you hard numbers. But I can give you a window into what it looks like mathematically if it follows the old Personal Loot design.
Each unlocked person contributed a 20% chance at an item drop, so each group of 5 was a guaranteed item. A group of 20 would receive 4 items, a group of 10 would receive 2 items. A group of 13 would receive 2 items and have a 60% chance of getting a 3rd.
Adding locked players added people to the raid, but didn’t increase the loot percentage. So a group of 10 unlocked players would still get 2 pieces of loot, even if the total raid size was 30.
With that background established, a lot of raid leaders observed that bigger raid groups had more loot and only wanted to invite unlocked players. Many guild raids would prohibit their members from pugging so keep their loot pool large.
But it doesn’t really work that way. A group of 20 unlocked players has a 100% chance of 4 items. But what if one of them is locked? 19 unlocked players gets 3 items guaranteed, but the 4 item is the one that sent people into fits. There are 4 possible outcomes. The raid gets a 4th item and the locked player didn’t receive one (64%). The locked player received an item and the raid doesn’t get one (4%). The raid gets a 4th item and the locked player received an item (16%). The raid doesn’t get a 4th item and neither did the locked player (16%).
Adding all this together, the chance that the raid gets at least a 4th item is 84%, with 16% of that actually being a 5th item for the raid because of the person who pugged early. However, there is a 16% chance that the raid only gets 3 items. Taken over enough iterations, those two 16% chances will average out and the amount of loot per boss will remain 4 for the group of 20, regardless of who is locked and who isn’t. The numbers change, but the principle is the same for more locked players.
If Group Loot uses the same proportions, then it is always best to just run your raid with the people you want to raid with, and not worry about the drop numbers, because the math doesn’t really change unless you’re able to convince a bunch of unlocked pugs to come and not roll on loot.
If there is a static number of drops for each boss, regardless of raid size, then the fewer unlocked players you bring the better, but I would be shocked if they allowed something so easily exploitable to exist in the game. You could funnel X number of items to a single unlocked player for every boss, which would be insane.