Add the User Defined Function StringConcat located on this page (link) into your sheet. The code is at the bottom and should be placed in a Module.
http://www.cpearson.com/excel/stringconcatenation.aspx
Then just put US1, US2 ad US3 into 3 cells and enter the following array formula
=StringConcat(", ",IF($A$2:$A$15=C2,$B$2:$B$15,"")) and copy down. That formula is an array and must be confirmed with CTRL-SHIFT-ENTER, not just ENTER.
A workbook sample is attached.
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