The addition of nearly 2 million SEIU workers would bring the total number of AFL-CIO members to almost 15 million. The Service Employees International Union is returning to the AFL-CIO after 20 years, betting that a united labor movement will do more to help workers overcome legal challenges to organize and join unions.The union groups’ executive boards each unanimously approved the combination on Wednesday, with plans to formally announce the affiliation at a Thursday roundtable discussion in Austin, with workers who are attempting to become union members.