用 joined, joined in 填空：

Soon some of the teachers ______ the singing.

Some of us decided to _____ the strike.  

Everyone there ______ the conversation.

【答】《薄冰英语惯用法词典》指出：join 意思是“加入（某项活动），可与 in 连用。...... We shall be glad to join (in) the discussion . 我们将很高兴参加这项讨论。

所以，这几道题填 joined 或者 joined in 都正确。