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In my textbook, there are instructions for a composition. These instructions
are the following: こんど、国際交流協会で座談会があります。テーマは「文化や習慣の違い」です。
下のことについて、みんなで座談会の観衆尾をしましょう。

Then, a list of further specifying instructions follows, specifying various
topics to talk about. One of them is the following:

で話したことをまとめてメールで送りましょう It is this で at the very beginning which I have never
seen so far ^^ Mostly because I've never seen particles taking the lead
position in a sentence xD Here's my attempt at translation. => "Please send
the talked about things all in one piece with E-mail."

EDIT: Since some asked for more context, I uploaded a scan of the full
exercise. <https://www.docdroid.net/hBn7YgR/1.pdf> Its number 5, チャレンジしましょう.
CC BY-SA 3.0
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grammar
what is this で in front of はなした doing?
Building on l'electeur's comment, it is likely just で which precludes a
sentence that supposedly builds on the previous statement.

> 先生はもう帰りました。で、どうしましょうか？ Professor already left. So, what should we do?

In your case, the instruction just tells you that after observing the
roundtable discussion you should all gather and summarize your thoughts and
email them.
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Now that you posted the link to the PDF, it's obvious that
友達や先生に１）で話したことをまとめてメールで送りましょう means "Let's send emails to your friends and
teacher about what you discussed in #1". The で is marking section #1 as where
the discussion took place.
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