Nearly 600 mechanics and related workers at Atlantic Southeast Airlines last month voted to join the Teamsters union by a 2-to-1 margin. The ASA workers became the first to win union representation under new rules instituted by the National Mediation Board this year. The new rules now allow a work group to unionize if a majority of those casting ballots vote in favor of union representation. Before the rule change, a union needed to secure “yes” votes from the majority of the entire work group, including those who did not vote.