Several people behind the same NAT router can play SP together, is this what you mean?
If so, each must have a separate IP address. Up to 8 can be joined together.
Several people behind the same NAT router can play online, as well. PM Chaos and explain
How many machines are behind the same router
Who they are (account names)
How are they related (friends or siblings playing in the same house)
I can give a more helpful explanation when I have more information (from you) about what's going on.