Format the reputation

JavaScript (ES6), 76 68 bytes

x=>x<1e4?x.toLocaleString():(x<1e5?(x/1e2+.5|0)/10:(x/1e3+.5|0))+"k"

Another first attempt. Thank goodness for that handy .toLocaleString(), the shortest alternative I could find is 21 bytes longer...

This separates thousands by either , or ., depending on what country you live in. For five two bytes more, you can make it always use a comma:

x=>x<1e4?x.toLocaleString`en`:(x<1e5?(x/1e2+.5|0)/10:(x/1e3+.5|0))+"k"

Japt, 50 48 bytes

First attempt; there may be a better method.

U<A³?U:U<L²?Us i1', :(U<1e5?Ue2n)r /A:Ue3n)r)+'k

Try it online!

How it works

          // Implicit: U = input integer, A = 10, L = 100
U<A³?U    // If U is less than A³ (10³ = 1000), return U.
:U<L²?    // Else, if U is less than L² (100² = 10000), return:
Us i1',   //  U.toString, with a comma inserted at position 1.
:(        // Else, return:
U<1e5?    //  If U is less than 1e5:
Ue2n)     //   U * (10 to the power of -2), 
r /A      //   rounded and divided by 10.
:Ue3n)r)  //  Else: U * (10 to the power of -3), rounded.
+'k       //  Either way, add a "k" to the end.
          // Implicit: output last expression

JavaScript (ES6), 71

Beating @ETHProductions while he does not see my hint. He saw it.

x=>x<1e3?x:x<1e4?(x+='')[0]+','+x.slice(1):(x/1e3).toFixed(x<99950)+'k'

Test

f=x=>x<1e3?x:x<1e4?(x+='')[0]+','+x.slice(1):(x/1e3).toFixed(x<99950)+'k'

function test() { n=+I.value, O.textContent = n + ' -> ' + f(n) }

test()

<input id=I type=number value=19557 oninput=test()>
<pre id=O></pre>

Test