Comma separated values

Hi,
I have list of users belongs to each city, I want to autopopulated my summary sheet using formulas
Summary sheet includes City name and Comma separted User list.

I have attached sample excel workbook in that Sheet1 is summary page and Sheet2 is actual data entered/extracted from database.
Can anyone please look into and tell me which formula I can use? or If you have sample formula that solves my problem please share with me.

Thanks,