協 (Kanji for cooperate)

Onyomi

KYOU	We cooperated TODAY (今日= KYOU)

There's THREE 'strong' radicals, so you can say:
Three strong people cooperating have the strength of ten people working alone.

協力(きょうりょく)

cooperation ★★★☆☆
協 (cooperate) + 力 (strong) = 協力 (cooperation)

妥協(だきょう) する

compromise ★★★☆☆
妥 (compromise) + 協 (cooperate) = 妥協 (compromise)

協会(きょうかい)

union ★★☆☆☆
協 (cooperate) + 会 (big meeting) = 協会 (union)

a society or association - but usually used about labor unions.