I am the GM of guild FUBAR on s3.Nightmare; at the end of the weekly battle I divide the gained refine stones (and wep) between the members that participated. Since no other option is available in game we do it through the market.
It went fine the first few times but since it is now known how we go about sharing the 'loot' we have had several shares intercepted.
Currently the Guild Vault option is not implemented in the game, not knowing the specifics of the vault but (judging by the description: "Donate items in vault to increase your contribution") I am assuming that the vault will be accessible to all members. Perhaps it is an option to use the guild vault to be able and share loot with the guild members, plus it will give the option for members to exchange items that way without having to use Diamonds.
An other option would be to implement message capabilities in game with attachment. We now only receive 'system mail', but if player to player mail with attachments was available it would make things a lot easier for a lot of players.
The existing 'system email interface' can be expanded upon, it would mean a bit more congestion I'm sure; but with a '3 day expiration' for opened mail and perhaps a '14 day expiration' for unopened mail, that congestion can be somewhat controlled. Or to enable 'trade' between members. Though since no foundation for that is currently existing in game it would require more of an adjustment than the previously listed options.