English translation: Cooperative members must take part...
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Explanation: Members of the Cooperative must take part in business activities of the Cooperative in the volume constituting major part of yearly volume of the activities that have been cooperated.
Sounds clumsy but russian in this sentense is quite terrible!
The main idea of this passage is that members of the Cooperative must take part in most of the yearly business activities that have been cooperated under this Cooperative. Thus, большая часть годового оборота is in volume terms.