A large POST sent from the form rendered by /co_group_members/select/cogroup:<id>, which can happen when the number of memberships in the group is large, will cause the following error in the Apache error log file:
PHP Warning: Unknown:
Input variables exceeded 1000. To increase the limit change max_input_vars in php.ini.
Since PHP truncates the incoming POST CakePHP sees that as tampering and sends the data to the blackhole.
Increasing max_input_vars can help but a better solution is to change the form/view so that under normal circumstances the POST sent upstream does not need to be so large.