Firstly, you have to consider who you are your family and friends are selling the shares to. You either have
- find a buyer before-hand. If that happens, then the trade will have very little effect on the market.
- Sell to dealers/brokers. In this case, the brokers are going to make sure that they don't lose money and will be sure that they will be able to sell the shares they buy at a profit. In other words, they won't give you a good price and you will lose money.
- Sell on the market when you can find buyers, which means lots of small trades made over time and the price will fall and fall. If you are selling a lot of shares, then buyers will be reluctant to buy until they think you have finished selling. So to encourage them to buy you have to sell cheap. In other words, you still lose money.
So this scenario is going to cost you and your family and friends a lot of money.
If you do start a large sell off , what should happen is that the share price starts to fall. Then, if this is a large publicly traded company, when the share price falls to below a certain level, other people will perceive it to be a bargain and start buying up the stock and the price stabilises.
What can happen is that that people start to wonder why the stock is being sold and begin to wonder if the sellers know something bad about the company. (That would probably be insider dealing, but it happens). So other people start selling as well and the price falls further.
Even then, there will be some people who know the true value of the company. Sometimes you get Company Directors buying shares as a show of confidence in the company and sometimes the company starts buying up their own shares. So the price stabilises.