na-adjective
Describes something done by or involving a group as a whole, often with a nuance of shared responsibility or action.
集団的な決定には時間がかかる。
Collective decisions take time.
彼らは集団的な責任
They feel a collective responsibility.
共同 means 'joint' or 'cooperative', emphasizing collaboration, whereas 集団的 focuses on the group as a unit.
Formed from 集団 (group) + 的 (adjectival suffix). The suffix 的 derives from Chinese and attaches to nouns to form adjectival nouns.