In connection with the Artists at Dartmouth Exhibition, the Hopkins Center, in cooperation with the Dartmouth Alumni Association of Greater Boston and the Mayor's Cultural Affairs Office, is sponsoring a cocktail party in Boston's New City Hall, 6 to 8 p.m. on October 22, the evening before the Dartmouth-Harvard football game. Tickets at $10 per person may be purchased by writing to H. Allen Dingwall Jr. at Hopkins Center, Hanover, N. H., or telephoning him at (603) 646-2164. Tickets must be purchased in advance because City Hall regulations do not allow cash transactions inside the exhibition hall.