Two ways that I can think of.
1) Have the grid inside an ASP.NET Update Panel and refresh this after closing the popup.
2) Use an AJAX query to retrieve data and refresh on the client-side.
This should help with #1,
http://weblogs.asp.net/jeffreyzhao/archive/2008/04/26/refresh-the-updatepanel-using-javascript-code.aspx[
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#2 could be more complicated depending your circumstances.
If you are using, or can use, ASP.NET 4.0, this might be an option,
http://msdn.microsoft.com/en-us/magazine/ee819084.aspx[
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