"Is party to" is a phrase that is commonly used in legal documents and agreements to refer to someone's involvement in a particular matter. However, there are many synonyms for this phrase that can be used to add variety and clarity to your writing. Some alternatives include "participates in," "has a stake in," "is involved with," "is implicated in," "is included in," "has a hand in," "has a role in," and "is an active participant in." By using different synonyms, you can make your writing more interesting and engaging while still effectively communicating the same meaning.