WHAT can young people in Hongkong and on the Mainland do for the economic betterment of their respective societies? Answers to that question will be suggested at a joint Hongkong-Mainland conference called ''Economic Development and Youth Participation in the 21st Century'', organised by the Hongkong Federation of Youth Groups. Several distinguished persons have been invited to address the gathering. Problems young people face, and the task of matching youth work and youth education will be some of the areas for discussion. The conference will be held on June 10, 12 and 14 at the Hongkong Convention and Exhibition Centre. The admission fee for the three-day event is $150 (including an information book and conference proceedings). For more information, call 395-0161 or 564-7604.