The following constructions using party are commonplace:
Acme is a party to a confidentiality agreement with Widgetco dated October 7, 2007.
Acme and Widgetco are parties to a confidentiality agreement dated October 7, 2007.
In the above examples, party is used as a noun. I suggest that in this context it would be preferable to use it as an adjective. That would result in the same examples being revised to read as follows:
Acme is party to a confidentiality agreement with Widgetco dated October 7, 2007.
Acme and Widgetco are party to a confidentiality agreement dated October 7, 2007.
I have yet to see any mention of this distinction in a dictionary, but I’m comfortable with my preference for using party as an adjective.
I am unclear as to why using “party” as an adjective is superior to using it as a noun.
Kazu: If you can accomplish any given drafting goal in two or more ways, you should choose, and use consistently, the usage that’s clearest and most efficient. In this case, I suggest that using party as an adjective is more concise than using it as a noun. I’d be the first to admit that the difference is trivial enough. But clear drafting is, in large measure, the result of the cumulative effect of many such choices. Ken