In order not to lose your entire deposit, you do not need to open an order for 100% of your available funds. Actually, there is a risk management strategy for this. Try trading 2-3% of your deposit. In this case, if the price moves contrary to your expectations, you will lose only these 2-3%, and not the entire deposit.
Well if they start with only 100$ then this method would mean spending 2-3$ per trade which is literally nothing. Therefore proper choice coins before starting the trade is important. The altcoins will always lead to bigger losses but not bitcoin, so that is one point. Again knowing how to deal with a opposite side movement is also important. Here you don't give up hope and panic sell but you hold it so that when the price crosses over you can sell the asset to break even.
However these little things will never be learnt from signals groups and all. It comes from practice and experience.
I do agree that trying to trade with such a small amount wouldn't work, a 2-3% method would work if you have at least 10k dollars, because in that case it is 200-300 dollars, and you could make some money there, but even in that situation if you make as much money as doubling your money, that would be 300 dollars more to your 10k, nothing big right away, it has to be consistent and for a long term in order to say that you did well.
I believe if you have just 100 dollars, then the lowest you should go would be fifty bucks, because that means you will be able to make a lot more, something that actually means something. Once again, even with just hundred bucks, you are going to end up with consistent and long term results that would mean something, daily it won't be much.