In a small food co-op or buying club a separate committee may not be necessary as decisions can be taken by the whole group.
However, as an organisation grows, meetings of the whole membership may become harder to co-ordinate and the meetings themselves become unwieldy.
In this case you might want to elect a smaller committee to meet regularly and deal with the management of the food co-op, with more infrequent (generally annual) meetings of the whole membership.
Normally most committees will have particular officers including a chair, treasurer and secretary. These officers will be elected at the annual general meeting (AGM).
When recruiting committee members and electing officers you need to ensure that the people involved have enough time and commitment to take on board the responsibilities involved and attend regular meetings and also that they have the skills and knowledge to fulfil their roles.
The chair provides leadership for the organisation and ensures the committee fulfils its responsibilities for good goverance. Other tasks include authorising actions to be taken between committee meetings, and signing cheques and legal documents. It is good if they have leadership skills, tact and fairness, 'people' skills and also good knowledge about food co-ops.
The treasurer takes the lead in overseeing financial affairs and must ensure that the organisation is financially viable and that proper financial records and procedures are maintained. Their responsibilities include preparing and presenting budgets, accounts and financial statements, so you need to find someone who has some financial experience and business planning skills.
The secretary is responsible for preparing agendas as set by the chair and other committee members and circulating them and also writing minutes of the meetings. They may also make all the arrangements for meetings e.g. booking the room, arranging for equipment and refreshments. They therefore need to be quite organised and good at writing minutes.