If I’ve understood correctly, you have a list of items (let’s call them weapons) which are available to all Players. Each time you (as admin) add a new Weapon to the main inventory, you want to make the weapon available to all Players, including existing users for whom the weapons won’t have been available previously.
If so, I create data structure as follows. All Players would have “Weapon” and that field would connect to list of Weapons in your weapons inventory.
Every new user would get a “set list” of current weapons.
Every time I added a new weapon, I’d make changes to all existing players and set (effectively reset) their list of weapons to your current list of weapons.
If I’ve missed the point of your question, sorry, ignore me!