A conventioneer is a person who goes to a convention: a big meeting of people in the same job, party, or hobby, with talks, badges, and often booths. The word is most at home in American English, and newspapers like it for the crowds at political conventions or trade shows. It is a slightly journalistic word. In ordinary talk you might just say someone at the convention, or a conference-goer. The person is the conventioneer; the event itself is the convention. The ending -eer marks a person tied to that thing, the same kind of ending as in auctioneer.