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To start participating in something; to join. Come on, Davey, join in the fun. Just because the other kids were making fun of him doesn't mean you should have joined in.
join in (with something)
to participate in doing something. The older boys joined in with the singing. I'm glad they joined in. We needed basses.
To take part in something; participate in something: The tenors started singing, and then the sopranos joined in. The adults joined in the celebration.