The most common and most likely answer to this crossword clue is the 4 plus 4 letter word TAKE PART
T
A
K
E
P
A
R
T
Buddy explains!
The straight part
We know that "to join in" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case to join in = take part.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.