According to Google Known Sites section of knowledge Graph, if you add a markup you will get your company showing in SERP Knowledge Graph. Well, can someone please elaborate what is consider as Known Sites? is it someone who verified his/her site with a webmaster? or a site that holds the most authority?Google Statement:Known sitesA known site is the official website for an entity as  shown in Knowledge Graph cards. If you add markup to a known site,  that data is treated as authoritative and used for Knowledge Graph  cards where it is not already determined by data you put into Google  My Business. This includes your official logo and social profile  links. However, if you enter contact information through Google My  Business, that data source is treated as authoritative. We recommend  you keep the data sources in sync if you use both types. Read the  guide Enhance Your Site's Attributes
What is considered Known sites in Google Knowledge Graph?
google search console;serps;knowledge graph
The answer is more or less in what you've quoted from Google:A known site is the official website for an entity as shown in Knowledge Graph cardsObviously, a prerequisite is Google's recognition of the entity in the first place. For most reasonably notable things that happens automatically, but it can be indirectly influenced by influencing Knowledge Graph sources, particularly Wikipedia, Wikidata (which superseded Freebase) and, for businesses, Google My Business.Once a Knowledge Graph card exists for an organisational entity, Google provides means to influence it via structured data (linked from the page you quote).