First, link the end of the sentence with the previous meaning.
The sports meeting is a festival that we can all take part in.
The sports meeting is an opportunity for everyone to take part in sports.
They danced and danced until many of us joined in.
They danced until many of us joined in.
Second, add something, which means "participate".
Everyone must attend the training.
Everyone must attend the training.
Can I take part in this game?
Can I take part in this game?
Join sb sth /v-ing means "with whom".
She joined our choir.
She joined our choir.
They join me in congratulating you.
They join me in congratulating you.
Extended data
Usage of conjunctions:
1, join as the solution of "connection" refers to connecting two independent objects by mechanical means (such as bonding, binding and welding) or traffic lines, which can be general objects (including buildings) or areas (including residential areas).
The characteristic of "connection" represented by join is that its object still retains independence to varying degrees, that is, its individual image still exists.
2. When interpreted as "join an organization and become a member of it", join is an instantaneous verb, so it cannot be used with phrases for a period of time, because the action it represents cannot continue. Join? Joining an activity can sometimes be used with adverbial of time to indicate continuation.
The present continuous tense of 3.join can indicate planned or planned actions, usually used with adverbials indicating future time.
4, because join itself contains the meaning of "together", so in general, it is no longer used with together, only used when it means "sticking together".