"Be party to" is a phrase that means to take part in or be involved in something. There are several synonyms for this phrase that can be used to convey a similar meaning. Some of these synonyms include participate in, collaborate on, engage in, contribute to, partake in, involve oneself in, and join in. Each of these synonyms emphasizes a different aspect of involvement, from active participation and collaboration to simply being part of a group or event. Using these synonyms can add variety and nuance to your writing, while still conveying the same basic idea of being involved in something.