Like Stephanie, I'd say they are synonym to a certain degree. To me, "to join in" focuses on the moment you start participating; "to take part in" can refer to that moment too, but also to the whole project. "I took part in the school fair organization, it was a big success." vs. "I joined the school board, they were very welcoming". Also, I would only use "join IN" on its own ("The school board needed someone, so I joined in.") while you need to state what you "take part in": "They lacked volonteers, so I had to take part in the school fair organization".

Join = to link or connect (Verb) Example: Please, "join" the charger to the cell phone. To join = an action Example: I plan "to join" my friends at the park. Take part in = to be involved Example: I will "take part" in cooking dinner. The words can be interchanged to some degree. They are synonyms. Example: Baseball Game Today, I will "join" my friends for a baseball game. Timmy asked me to "take part" because they needed a pitcher. I was happy when he asked me "to join" in. I hope this helps you.

A person would "join" a group. It means to become a member, like to join a club or organization. To "join in" means to participate. "Come join in the fun." To "take part in" also means to participate or to share. "Take a part in the discussion."