If people gather and do a voluntary social work together, say on Sundays, they gather at their job places and do different cleaning work, what is the activity for English? Is there a single word for this sort of job?
Last fall, about 80 of my coworkers got together on a Saturday and planted trees at a local park. We called it a day of community service or community outreach.
You mentioned cleaning - do you mean they cleaned their own office? That would be "additional work without pay" and wouldn't go over very well here.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.