Since we are more of 'friends with benefits' than SO's of any flavor, probably not required.
If you wanted to be painfully correct, inviting Sarah would be appropriate, as she's essentially our third. however, as we don't all live in the same house (which is where i personally draw the lines for this sort of thing) it's not strictly necessary.
thanks for asking: )
So there we have it.
I still ordered Steve's book, though.