To 'be party to something' means to be a part of it. If you are party to something you are aware of what is going on or you are a participant or a contributor to something. To be a party to something can be another way of asking someone about their involvement in a deed or a project. This is quite a formal expression used often in legal contexts.

1. I think we'll get a lot of money when we auction this item. We have a lot of interested parties.
2. When he failed to secure a seat in the election, he started up his own political party.
3. I didn't know that was happening, because I wasn't party to that information at the time.
4. I'm not sure who told my friend, but when he told me, I knew it was third party information.
5. My car insurance is so expensive, and it's only Third Party, Fire and Theft cover. It's not even Fully Comprehensive.

1. Anyone who was party to the breach in security needs to report their knowledge to HR immediately, we are aware that some peoples personal information and salaries have been disclosed to others.
2. I made it clear that as a company we refuse to be party to anything underhand.