Do something like this

(function executeRule(current, previous /*null when async*/ ) {

    if (current.getTableName() == 'sn_customerservice_managed_service_case') {

        var result = [];
        var currentUser = gs.getUserID();

        var rec = new GlideRecord('customer_account');
        rec.addNotNullQuery('u_access_container_group');
        //rec.addQuery('u_managed_services',true);
        rec.addQuery('customer', true);

        rec.query();

        while (rec.next()) {

            if (currentUser.isMemberOf(rec.u_access_container_group.name.toString())) {
                result.push( rec.sys_id.toString());
            }
        }
     
       current.addQuery('company', "IN", result);
    }
})(current, previous);