Can I invest $100 in Bitcoin?

Absolutely... you can invest a penny in bitcoin if you really wanted to... doesn't mean you're going to become a millionaire though.

Invest what you can each week or month, keep at it, and keep building your stash.

Bigger institutions are jumping on board every day now. Seeing bigger and bigger firms get involved, and legitimate analysts predicting 500k - 1mil EACH within the next 15-30 years.