just because this answer might be helpfull for many others as well, I'll write it all down here:
People can play the game from the same PC, but then there is an additional rule to follow. Teams playing from the same PC will never be able to transfer players to each other (well they technically can, but they should never) . This might seem a hindrance, and at times it will be a little anoying, but the transfermarket is prety big and a likewise player can be found with some patience. Moreover this is the only way for us to tackle the traitors and cheater that slip in the game.
You can arrange scrimmages bewteen each other. We know it can be fun to challenge your relative, or friend's team, so we are not going to be harsh on that point.
A rule not only for those who play from the same PC, but maybe more tempting for those: do not log into each other's teams, for ANY reason. Each keeps his own password safely hidden, and is responsible to keep it that way. Often we discover people logging in to a friends account to do something he asked. This is called babysitting and is always sanctioned!
That's about the stuff you need to take into mind, for the rest the other rules apply just as for any other user.
They are not your friends; they dispise you. I am the only one you can count on. Trust me.