As simple as it might sound - the use of Company's, Companies and Companies' are still mistaken by many people due to sheer ignorance of the minor difference between them.
Where to use - COMPANY?
The "Company" is a (singular) collective noun, which means 1(one) company. Even though a company represents a group of people, in the eye of law it is considered to be a single legal entity (an artificial person).
Example - Apple Inc. is an American multinational technology company headquartered in Cupertino, California.
Where to use - COMPANIES?
The plural (collective) noun of Company is "Companies", which means two or more number of companies.
Example - The Standard & Poor's 500 (S&P 500) is an American stock market index based on the market capitalizations of 500 large companies having common stock listed on the NYSE or NASDAQ.
Where to use - COMPANY's?
Company's - a singular possessive noun. It means something belongs to a Company viz. assets, employees, etc.
Example - The Company's workers went on a strike in pursuit of a pay increase earlier this month.
Where to use - COMPANIES's?
Companies's - a plural possessive noun. It means something common belongs to a number of Companies.
Example - Record Companies’ U.S. Revenue Up 8.1% in First Half of 2016.