You can still JOIN unions while this event is ongoing. You just can't leave or be kicked from one. Too bad that you count as inside a union though, it's gonna be extremely hard to unlock alot of the rewards. Plus side is there's no SSR's that you miss out on though.
If other people are union-less right now, definitely go find an active union ASAP! I'm sure unions with vacancies are all desperately trying to fill their ranks (Judging by the amount of Scouts people have been spamming mindless for the last week)