합세해요 · 합세했어요 · 합세할 거예요 · 합세하세요
to join forces, to team up
Example
주민들과 학생들이 합세해서 공원을 청소했어요.
Residents and students joined forces to clean the park.
Word parts
합 combine+세 force
Literally 'combining forces'.
Good to know
가세하다 = join an existing side; 합세하다 = groups pooling strength together.