While in a such property, players from the same crew will be able to discuss maybe tactics, strategies, etc. /crewchat and /cc will allow crewmembers to join in a same chat.
The chat will be private, meaning that nobody, but you and your crew members are able to see it. To cancel this, you and your can type /cancelcrewchat or /ccc
For this thing you'd need to define crew members, which would further need crew system to do so (i mean, what if there's someone in the house who isn't part of the crew)
For a gang safe with extra ammo storage slots, you'd kind of still need crew system, since even individuals could buy it for themselves. In that case it should be expensive, but people don't use houses that much (at least i don't so assuming...)
A simple "/givehousekey [ID]" could be a nice suggestion, but to make things more interesting, maybe add anyone with house keys can use your safe (so you limit keys only to trusted people, *optional)