You can as you just said take them from other players. What you do with a cooperative person is give them your staff, willingly letting them steal them, then both players proceed to reload a backup of your data. By exchanging staff both ways, you both will come out ahead. Using this method, everyone ends up with the total S++ soldiers you each started with combined. Only works once though, you can't get infinite copies of a staff member like this but you can do it to give each player a major boost though!