組織 and 団体: organizing and a group acting together
Compare organizing an investigation team with a group cleaning a riverbank or using a reservation service. Read the perspective on the group rather than defining the words by size.
An organization and the act of organizing
組織 (そしき) can mean an organization or organizing. 「小さな団体が全国的な組織へ成長した。」 describes a small group growing into a nationwide organization. That change does not define the size of every 組織.
「担当者は新しい調査班を組織した。」 says the person in charge organized a new investigation team. 組織した identifies an act of forming the team rather than only naming an existing group.
A group acting or using a service
団体 (だんたい) refers to a group or organization. 「地域の団体が川辺の清掃活動を行った。」 identifies a local group as the actor cleaning the riverbank.
「十人以上の団体は事前の予約が必要だ。」 states an advance-reservation requirement for groups of ten or more. It describes how the group is treated by the service, not a completed reservation.
Two perspectives on a group
Organizing the team emphasizes its formation; the cleaning group is the actor in a shared activity. An organized group can be described with either expression, so large versus small is not the defining distinction.
The nationwide growth and the ten-person reservation condition belong to separate examples. Neither number becomes a universal definition of an organization or group.
Must a small group be 団体 and a large group be 組織?
Show the answer and why
No. The growth example describes one group's change. Read the act of organizing and the group acting together or being treated as a group, rather than choosing solely by size.