I was wondering if Squad will have a way to share ammunition, whether its magazines, grenades, or anything else, and then I started thinking about the game Ground Branch's functional MOLLE webbing system. It allows you to place magazines, grenades, first aid, or whatever on selected hardpoints, and that effects what you have in game and how long it takes to get that item depending on where it's placed. I understand that Squad is trying to be 100% authentic woth things such as the gear each faction has, but atleast for the US Army I'm sure the placement of individual pouches isn't regulated as much as what you carry. So I assume it could work, just with restrictions based on your role.
The second option is having an inventory system like Arma. The load of how much you carry would be dependent on your class/role, and you would be able to access other people's inventory (within reason to prevent griefing or trolls) to let you share ammo.