How to use 合う as a Compound Verb

合あう by itself means to fit, match, correspond or something similar to that. It can also be stuck to the end of other verbs to create compound verbs.
https://japaneseuniverse.com/2024/02/18/pac-man-in-japaneseconcepts-behind-development-to-global-icon/
In a compound verbs, most of the time the first verb has most of the meaning. The second verb adds extra information to the first verb.

合う as a Compound Verb

合う is common as a compound verb.  It adds the meaning of doing something together, mutually, or with each-other to the verb it follows.

話はなす　＋　合う　→　話し合う
talk + together →　talk together, discuss something

助たすける　＋　合う　→　助け合う
help + each-other →　Help each-other

向むく　＋　合う　→　向き合う
face + each-other →　Face each-other

Resources

• 合う