合う【あう】

合う goes on the ます-stem of a verb and makes the action mutual: two or more people do it to each other, or all put something into it together. The compound conjugates as an う-verb: たすけあう助け合う, たすけあいます助け合います, たすけあった助け合った, たすけあわない助け合わない.

It takes the ます-stem, never the dictionary form or the て-form.

• × 助けるあう → 助け合う
• × 話して合う → 話し合う

Who does it with whom

The people involved can all be the subject, grouped with たち, みんなで or おたがいにお互いに. Or one person is the subject and the other takes と. An object keeps its を.

合う or 一緒に

いっしょに一緒に only says that people act side by side. 合う says the action passes between them.

Compounds with their own meaning

A few 合う compounds are everyday words whose meaning has moved away from "each other", and they are best learned one by one. しりあう知り合う is "get to know", つきあう付き合う is "go out with" or "keep someone company", and むきあう向き合う is "face" – a person, or a problem you can't avoid.