Your code mixes dynamic/client and standard record access mode.
For a user event before submit script you don't need the insert call. Just:
var newAt = nlapiGetLineItemCount('attendee') + 1;
nlapiSetLineItemValue('attendee', 'attendee', newAt, '95001');
For a user event after submit script similar but:
var eventRec = nlapiLoadRecord(nlapiGetRecordType(), nlapiGetRecordId());
var newAt = eventRec.getLineItemCount('attendee') + 1;
eventRec.setLineItemValue('attendee', 'attendee', newAt, '95001');
//add more?
nlapiSubmitRecord(eventRec);