The Short Answer: Anyone!
Any worker can form or join a union at its most basic: a group of workers who take collective action to win material changes in their workplace. You don't need to work in a specialized industry, make a certain amount of money, or be a certain kind of worker.
Even if your place of work is not unionized you can join (or make) a union (just tell DON'T your manager). They will give help and support if you have problems including stuff outside work, like if you have brought a shoddy product their lawyers will help in getting recompense.
You insure your car; you insure your house; you insure (sometimes) your life, and you don't insure your job???