Using the Suffix 同士 to Group Entities Together (NOUN + 同士)
The noun word 同士 (doushi), which translates to “fellow,” “mutual,” “companion,” “comrade,” etc., can be used as a suffix to another NOUN word, X, to group multiple Xs together. This expresses that these Xs do something to, for, or against each other. Translation depends on context, though common ones include: “each other,” “together,” “between,” “among,” “with,” etc.
NOUN word + 同士 + (が/で/を/の) + X
(e.g. 友達同士, 学生同士, 男同士, 恋人同士, 仲間同士, etc.)
Otoko doushi no kyoukan
“Empathy between men”
Nakamadoushi de kono jiken ni tsuite hanashiatte kimatte kudasai.
“Please discuss this matter amongst yourselves as fellow colleagues and come to a decision.”
Kono daigaku wa, zenkoku no mottomo yuushuu na gakuseidoushi no senjyou da!
“This university is a battlefield for the fellow brightest students across the country!”
Kono aida hatsu okyuuryou ga haittanda. Kashitoite ageyouka?
“I just happened to get my first paycheck the other day. Shall I lend you some money?”
Ii! Tomodachidoushi no kashikari wa yuujyou no hatan no genin nandayo!
“N-No! Lending and borrowing money between friends is what leads to ruined friendships!”
*This example is taken from the anime New Game! episode #06.