〜合う
(do) to each other, together
Verb ます-stem + 合う
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合う after a ます-stem makes an action mutual: 助け合う, "help each other"; 見つめ合う, "gaze at each other"; 話し合う, "talk it over together".
It's a very productive pattern, and some combinations have become words in their own right: 話し合い ("discussion"), 付き合う ("go out with, keep company"), 知り合う ("get acquainted").
With 出す, it means everyone contributing: 意見を出し合う, "share ideas"; お金を出し合う, "chip in".
It conjugates like 合う itself, a godan verb: 合った, 合って, 合いましょう.
Compare てあげる and てくれる, which describe a favour going one way. 合う makes it two-way, so it's a natural fit for friendship, teamwork and negotiation.
Examples
困った時は、助け合うことが大切だ。
When times are hard, it's important to help each other.
二人は見つめ合った。
The two of them gazed at each other.
みんなで意見を出し合った。
Everyone pitched in with ideas.
問題について話し合いましょう。
Let's talk the problem over together.
私たちは毎日連絡を取り合っている。
We keep in touch with each other every day.