Hi Helen! Good to see you again. :) "join in" to take part in an activity. "To join the party" means to be(come) part of the party, "To join IN the party" means to take part in the activities of the party. I hope this simple example explained it for you.

I can't give you a exact rule or grammatical explanation. After the word "join" I expect to hear a noun naming a group, person, or organization. "A few years ago I decided to join a barbershop chorus." "After work, join me at Starbucks for coffee." After the phrase "join in" I expect to hear something that names an action or activity--often, the -ing form of a verb. "Please join me in singing 'Happy Birthday' to Tom." "Please join us in our celebration of the holiday season." "I joined them in wishing 'bon voyage' to Linda on her trip to Mexico."