What is it called when a group of people all buy a certain stock then hold it to artificially raise the price?

Question

What is it called when a group of people all buy a certain stock then hold it to artificially raise the price?

Answer 1

back in the 1920's it was called a pool operation.  It is pretty much unheard of today for the most part.  One reason that is that is much more difficult to do today.  There is a much larger float of most stocks requiring a much larger capital base to manipulate the stock price.  There has been recent speculation that it has been done by short sellers selling shares of the financially embarrassed companies such as the major banks and brokerage firms.  If so it was done not by a group of people so much as by a group of financial firms with the ability to short thousands if not millions of shares of these companies.  Stocks do tend to drop much faster than they tend to rise. And the companies that it was done to probably deserved it.  

Answer 2

I believe you are thinking of 'pump and dump'

People buy a lot of a penny stock then flood the emails with spam messages recommending it as a "strong buy."   After enough suckers buy, they dump it and the price drops back below its previous low.