conventionist

plural conventionists

1. One who enters into a convention, covenant, or contract.

   1 more example

   It must needs be a hostile kind of a world, when the buyer (if it be but of a sorry post-chaise) cannot go forth with the seller thereof into the street to terminate the difference betwixt them, but he instantly falls into the same frame of mind and views his conventionist with the same sort of eye, as if he was going along with him to Hyde park corner to fight a duel.

   1768, Laurence Sterne, A Sentimental Journey through France and Italy, London: T. Becket & P.A. De Hondt, page 40

2. One who supports a convention or assembly.