合う

合う in this sense expresses reciprocal action or cooperation between people. It attaches to the stem of a verb to mean that both sides do the action for/with/to each other. The nuance is collaborative or reciprocal rather than a one-sided action. It can also appear in compounds where two parties cooperate or reach mutual understanding. In everyday use, expect to see 多くの動詞の語幹 + 合う (e.g., 助け合う, 話し合う, 分かり合う). You can also emphasize reciprocity with お互いに to make it explicit that the action is mutual.