Final answer:
Without additional context, it is impossible to determine the year Illinois reached its all-time membership record or what the membership number was. Further information on the specific type of membership mentioned is necessary for an accurate response.
Step-by-step explanation:
The question is asking for the year in which Illinois (IL) reached its peak in membership in a certain context, possibly related to an organization or club. Unfortunately, the context is unclear as the term 'membership' could refer to various types of organizations or associations.
Without a specific reference to which 'membership' is being referred to, it is impossible to provide an accurate year or membership number. If this is about a historical organization, a sports club, or another form of the group established in Illinois, further information would be necessary to pinpoint the all-time membership record of the specified entity. It would also be relevant to know whether this record is a current statistic or one that was recorded in historical records.