The examples they give always say "you defeat a boss and a coffer appears", so I think it's safe to assume that each boss will immediately drop a coffer for each full party.
I guess the first week or two there may be a lot of people in for the freshmeatbosses who stay till the end. Otherwise people who have BC static will have no reason to stay once they have their loot for the week.
If Crystal Tower really is as easy as they say, it's probably better to have static BC teams break up and each join CT solo. This way they can each drop out after getting their loot. If a full static join as one, then not only will they be fighting against each other (i.e. it will take at least 2 full runs for the whole static team to each get a drop), they will also have to wait for the other parties to replace people.
Once everyone's done a solo join in CT, then they all get together and join as one party, because then only part of them need loot, which now makes a premade static more efficient than joining solo.
...just wondering out loud. Hope I'm wrong about this. O.Oa