From Longman Dictionary of Contemporary Englishjoin in (something) phrasal verbTAKE PART/BE INVOLVEDto take part in something that a group of people are doing or that someone else does In the evening there was a barbecue, with the whole village joining in the fun. He stared at them without joining in the conversation. He laughed loudly, and Mattie joined in. → join→ See Verb table
Examples from the Corpus
join in • He sometimes watched, apparently amused and distracted, but he never joined in.• Shrieks of delight at this game which everyone joined in.• Kloss then put the eggs on the dead miller's body and forced the wife to join in the meal.• This in turn is credible because if any other firm does not join in this punishment it will be punished, and so on.• People spontaneously join in when they hear some one else laughing.• About 50 fans poured on to the pitch to join in with the players.From Longman Business Dictionaryjoin in something phrasal verb [transitive] to take part in something with another group, company, or organizationWe would be eager to join in projects of that sort. → join→ See Verb table 