Union membership rose by 100,000, or 0.6%, to 16.2 million in 1998 from the previous year, the Labor Department said. At the same time, the share of the work force represented by organized labor declined to 13.9% from 14.1%, reflecting population growth and an overall expansion in the work force, the department said. That is the least the percentage has fallen for the last five years. In 1994, there were 16.7 million union members, which reflected 15.5% of the work force. The AFL-CIO said the raw increase in membership reflects membership drives such as those that brought United Airlines workers into the International Assn. of Machinists in July.