You can invest at $100 but do not expect a higher profits due to a very low capital. But you can proceed with your investment whenever you have your additional capital.
Investment with fewer amounts and investment with more amounts for a wise and expert trader or investor will give two different results. Similarly trading with fewer amounts and trading with more amounts for a person having no knowledge of the market will give tow different loses. So it is up to knowledge if one has knowledge then investment with any amount is relevant while it is irrelevant for a person having no knowledge.