Find nth occurrence with multiple criteria

Hi Evreyone,

I am trying to write a VBA function that combines two elements:

1. Finding the nth occurrence of a value in a range
2. Use concatenate or & to combine multiple criteria

I really do not know where to start for writting this type of vba code. I have attached a excel work book where the first sheet is a duplicate of a similar workbook where a userform inserts new data rows. The second sheet is where I hope to combine rows that have an exact match for an ID code.

Thanks

Concat.xlsx