How to get the users of 50 kilometers radius from USERS Table using ZIP code in sql server

It'd be worth updating your question (use the green "Improve question" link to describe what it is the current query does (or post the query if it is not too long).

One thing to *try* to improve performance that sprung to mind immediately was to maintain latitude and longitude fields against zip codes. You can then calculate maximum & minimum longitudes and latitudes for the original codes (i.e. to form a "square"), then query ZIP codes in that square into a temp table. This is less computationally expensive than getting all ZIP code's distances from the start point, then deciding if < 50km. You can then run the query you have on the square results temp table (to get those in the radius) on a much smaller subset of ZIP codes.

I'm not sure how ZIP codes are organised (we have our own system in the UK) but there might be systematic things you can do to filter out candidates for example getting ZIP codes that border first, and traversing "borderers of borderers" until you get no results.

The other thing that I don't know if you need to handle is that ZIP codes aren't points (in the UK Post Codes represent streets, more or less) so I don't know whether you need to take into account the physical area of the code.