The most common and most likely answer to this crossword clue is the 9 letter word COHABITED
I've seen this crossword clue in The Times
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More clues leading to the result COHABITED
Buddy explains!
The straight part
We know that "Didn’t officially join" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case didn’t officially join = cohabited.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.
What does COHABITED mean?
COHABITED verb- To live together with someone else, especially in a romantic and sexual relationship but without being married.
- To coexist in common environs with.
- To engage in sexual intercourse; see coition.