As long as you are a worker — not an employer — you can join the IWW. Members of other unions (except officers), students, retirees, the unemployed, the self-employed, those in informal professions, and those unable to work may also join. To us, we are all workers.
There is a difference in interests at work between employees and employers. Even if someone doesn't own the business, if they have real hiring and firing power over other workers they are forced to serve the interests of their employer, Therefore, they cannot join the union.
Police officers and prison guards are also restricted from the union. Acting as the enforcers of wage slavery, law enforcement has a long standing history of suppressing workers through state sanctioned violence. Prison guards act similarly against our incarcerated fellow workers, who make up nearly 10% of our union.