Well, I installed BF2 and have been playing for a couple of days now, and my brothers want to as well. Works fine on my account but when they try and play online on their accounts, it says invalid cd key.
No. Every CD key is locked to the original (your) account, and can never be used again in other accounts. They will have to buy a fresh new copy of the game in order to be able to play on it.
You can share the account, but you can't both play at the same time. My buddy worked a later shift than I, so we both shared his BF2142 for a few weeks. I had my own soldier and everything, but then he changed jobs so I bought my own copy