I have seen people daven chazaras hashatz after a minyan has just ended out loud,and they expect everyone to answer their kedusha and then they continue the rest of their shmoneh esri quietly. Is there any mekor for this,did I miss something in the Shulchan Aruch?
Yes, there is a source: Shulchan Aruch Orach Chaim 69. They are doing what is known as "Perisah al haShema" whereby individuals who didn't hear Kaddish, Kedusha and Barchu can make it up with a later quorum. The details about how to do this are complicated and there are many opinions about what can and cannot be made up as well as how many of the people making up the later quorum need to have previously missed hearing those things. I highly recommend this article (starting about a quarter of the way down) which goes through many of the sources on the matter.