Reading around, and not to certain with the exact goings on.
I share a computer, so will have a single copy of the game, so will be beneficial for each person sharing to use their own account login.

When inputting the serial, will it be tied to an account that from then on will only be able to access multiplayer?
Or will multiple people be able to just use their own login for multiplayer, but the serial will still be tied to account just for authorisation purposes?

One more thing to add, does only one account get the bonuses, the one that redeems the code?