"To join a union" means to become a member of an organized group of workers who come together to protect their rights and improve their working conditions. Full definition
Part of the problem is that teachers join a union as if they were factory line workers not a professional association that demands quality from their members. (tnedreport.com)
This has been viewed by some as punishment for joining a union, and has seen employees struggle to meet their rent payments - whilst some have even lost their homes. (politics.co.uk)
In states that allow agency fees, more than 90 percent of teachers join a union, while only 68 percent join in states that don't. (educationnext.org)